DIY Overland Trailer Build Part 4: Cargo Box and Doors

In the previous installment in my DIY overland trailer build series, I walked through how I fabricated the trailer’s frame and assembled the axleless suspension and wheels. 

In this post, I’ll focus on the next stage of the build, namely, how I build the cargo box on top of that base frame. In addition to fabricating the frame for the box, I made doors for it and enclosed it with aluminum cladding. 

Here is the video that corresponds to this portion of the build:

Topics

• Overland Trailer Cargo Box Design

• Fabricating the Cargo Box

• Stabilizer Jacks

• Roof Rack Towers

• Overland Trailer Doors

• Overland Trailer Fenders

Overland Trailer Cargo Box Design

My overall trailer design called for a cargo trailer that could hold gear and supplies inside. The trailer would serve as a platform for a rooftop tent, shade awnings, and other camping amenities.

After perusing other overland trailer builds, I decided that two doors would be plenty for a door this size: a tailgate and a door near the front on the side. After looking at the height of overland fridges – something I wanted to add to the trailer down the road – I decided to make the box two feet high. This would provide plenty of space for most fridges (and for the Pelican Cooler I currently own) and any cargo cases I’ll use in the trailer.

Below are couple of my early design drawings, showing a side and rear view of the frame for the trailer box. I altered the design from this a bit later on, but here you can see the general dimensions and the location of the side door and tailgate.

My initial sketches of the cargo box showed it as a separate unit from the frame that would be bolted on once fabricated. I was thinking this would give me some flexibility later on if I wanted to change the size of the cargo box. After giving this some more thought, I realized I was overthinking it…

It made more sense to commit to an overall size for the box and weld the pillars onto the existing frame. Compared to my initial design, this reduced the trailer's weight by letting me avoid the need for separate metal framing at the bottom of the box.

Another way I cut weight was to use 1.5-inch square steel tubing for the box instead of the 2-inch tubing I used for the base trailer frame (what I initially planned to use, as you can see in the sketch above).

Fabricating the Cargo Box

Fabricating the cargo box was very similar to the process of making the base frame. The first step was cutting the frame members and prepping them for welding by removing the mill scale (a process I outlined in the previous post in this series).

To save time, I kept the wheels on the trailer frame while I assembled the cargo box. To make sure the uprights for the box were at right angles to the frame and all vertically aligned while welding, I carefully leveled the frame (adjusting the XO trailer jack) and then used welding magnets and a magnetic level to position them.

Before welding the uprights to the trailer frame, I used my angle grinder and a flap disc to grind down the welding beads on the top of the base frame. This allowed the upright members to sit squarely on the frame. It also made for a flat surface for affixing the aluminum floor to the frame, which came later in the process.

Stabilizer Jacks

Another element that I wanted to include in the build was trailer stabilizer jacks. Since we would be sleeping on top of the trailer, it was vital that it be stable and not rock substantially when we moved around in the tent.

There are a number of options for stabilizer jacks, including jacks that bold to the bottom of the frame and then crank down to connect with the ground and support the trailer.

While this style of jack is convenient, since you store them simply by cranking them into the up position, I didn’t like the idea of them reducing my clearance. Since they would be hanging from the bottom of the frame, my thinking is that they could get damaged or cause the trailer to get stuck in off-road conditions.

I opted to go with pipe-mount jacks with connection points that weld to the side of the frame (pictured above). These jacks retract up next to the frame or can be removed completely when not in use. The downside is that the jack mounts had to be welded to the frame.

I used Curt stabilizer jacks rated at 2,000 pounds, which is probably overkill for my trailer, especially considering I have four of them. That said, I hope to use the jacks to level the trailer while camping on uneven ground. We will be sleeping on top of the trailer, so I wanted them to be particularly sturdy.

Roof Rack Towers

At this stage, I also drilled holes for the roof rack towers that would eventually support the roof-top tent. I sourced the towers from Compact Camping Concepts (Dinoot). They are made of steel and designed to support the weight of a roof-top tent with people inside.

This step was fairly simple, requiring me to drill twelve holes, two for each tower. Later, I bolted Super Strut members to them to serve as the rack cross bars.

Overland Trailer Doors

Fabricating the doors for the trailer was a fairly time-consuming process and one where I made some mistakes. The main challenge was building doors that were square and fit well within the frame opening.

I built the square frame for the doors with 1-inch x 1-inch steel tubing and then skinned the door with steel sheet metal.

I cut the frame pieces with a chop saw and the sheet metal with a standard skill saw outfitted with a Diablo Steel Demon blade. These blades make it very easy to cut through metal – especially thin metal. I’ve found that they wear out quickly when cutting thick metal tubing and that a good chop saw works better.

One mistake I made was to make the doors a bit too big. While you want them to fit fairly snuggly, in my opinion, the doors should be a bit smaller than a bit too big. You can always use rubber trim to seal the space between the door and the frame, but it’s a pain to grind down the doors because they are too big (which is what happened to me).

Once I had welded the door frame, I laid the frame on the sheet metal and traced the outline of the frame onto the sheet. Then I used those lines to cut out the door cladding (a term I’ll use interchangeably with “skin” here). Similar to the trailer frame, I stripped the mill scale before welding.

To weld the door cladding onto the frame, I welded three-inch long beads at intervals on the inside of the door. My thinking was that welding the entire length would add a lot of weight and take longer to weld and that this dashed-line approach would suffice to hold the door on.

It’s a debatable approach, as it might have been simpler to just weld the seams entirely. Later, I used a sealant to fill in the gaps for waterproofing and then painted over the welding and the sealant. We’ll see how it works, and I’ll report back.

On one of the doors, I cut the skin a bit too wide and had to go back with the back with the skill saw and the angle grinder to clean it up. This was a pain and the next time I build a door like this, I’ll try to get the cut right the first time.

The major mistake I made with the doors was welding the skin onto the wrong side of the side door. Despite my best efforts, the area on the trailer frame where to door sits wasn’t completely square, so I had to adjust the door frame to fit.

But then I inadvertently welding the skin on the wrong side and the door wouldn’t fit in the cavity on the trailer frame. After trying a few different things, I opted to just install the door with the skin on the inside of the trailer. Not ideal. But, hey, it gives the trailer character!

After the doors were welded together, I drilled holes in the trailer box frame and the door frame and mounted the door on hinges. I used stainless steel Torx head security hardware to prevent evildoers from easily removing the door.

Once to doors were mounted, I installed latches for holding them shut. I like the look of toggle clamps and they are inexpensive and fairly adjustable, which makes it easy to mount them. Also, they can be locked closed with a padlock or cable lock, adding security.

One challenge part with the latches is that the screws that hold them to the trailer go through different thicknesses of metal in places. This required screws of different lengths. Also, the latches were wider than the 1-inch door frame, so the screw holes didn’t line up quite right.

For the side door, there wasn’t a good location to attach the latch, so I had to weld on an additional metal mounting plate (see below).

If I had to do it again, I’d probably add support plates on the inside of the tailgate for the latches to attach to as it would have avoided the issues with the latch being wider than the door frame.

Overland Trailer Fenders

The last major components of the trailer that I’ll cover in this installment are the fenders. The fenders turned out to be far more complicated than I expected.

This complexity stems from three things: 1) the fenders must be large enough for the oversized tires, 2) you have to account for the movement of the tires up and down due to suspension flex, and 3) you need to mount the fenders to the trailer somehow.

I’ll start with the tire size issue. I’ve come across a number of overland trailer build threads where they fabricated the fenders from scratch, presumably because they couldn’t find any pre-made fenders that would fit. I wanted to avoid the extra fabrication work, so was motivated to find some plug-and-play fenders. After one misfire (does anybody want a set of rusty unused fenders?), I discovered that Compact Camping Concepts made fenders that would fit the 35-inch tires I planned to use. These were a godsend.

The next step was figuring out where to mount them on my trailer. The Timbren suspension on the trailer has a deflection of 3 3/8 inches, meaning that at a maximum, the wheels will deflect that far upward in relation to the trailer frame when the suspension compresses due to bumps.

To make sure the wheels wouldn’t hit the fenders, which would be attached to the frame, I needed to mount them so that the inside top of the fender was floating at least 3 3/8 inches above the top of the tires. To make sure there was some clearance, I decided to mount them 5 inches above the tires to provide 1.5 inches of buffer.

The biggest challenge with the fenders was that the wheels stuck out from the side of the trailer’s frame. In fact, they stuck out so far that if I mounted the fenders directly to the side of the trailer, the wheels would stick out beyond the fenders. This would limit the effectiveness of the fenders, as they wouldn’t block rocks, mud, sand and other debris from being thrown by the wheels as well. I’d also read that to get a trailer registered in California, where I live, the fender has to completely cover the wheels.

My solution was to build a frame for the fenders that mounted to the trailer and held the fenders out over the tires. It was way more work than I’d anticipated putting in to mount the fenders, but still probably less work than building them entirely from scratch.

I bolted the fenders on with stainless steel hardware that fed through holes in the fender frame. The fenders came with holds pre-drilled in them, so I used those as the guide for the holes I drilled in the trailer frame.

One downside of this method of mounting the fenders is that it made cladding the trailer more complicated. The bracket I built to hold the fender to the frame obstructed my ability to attach a sheet of metal to the side of the trailer. My workaround was to attach the side cladding to the inside of the trailer frame. But this had knock-on effects, such as making it difficult to attach cargo tie-downs to the inside of the trailer and creating some headaches for weather sealing the trailer.

If I had to do it again, I might try to figure out a way to mount the fender brackets to the outside of the trailer AFTER the cladding had been added to the outside of the sides. Maybe by bolting on both the bracket and the fenders. As it is, the brackets and recessed cladding look kind of cool, so I’m not heartbroken.

Okay, that’s it for this edition. In the next installment, we’ll go through the process of painting and cladding the trailer.

All Overland Trailer Build Articles

• Step 1: Choosing a Trailer Style

• Step 2: Design Process

• Step 3: Frame Design and Fabrication

• Step 4: Cargo Box, Doors, and Fenders

• Step 5: Cladding, Trim, and Painting

• Step 6: Wiring Harness and Tail Lights