DIY Promaster Campervan Cargo Box Installation on Thule Rack: Affordable Space-Saving Solution

As I prepare for my summer road trip, I’ve become acutely aware of the value of every precious square inch inside my Thor Scope 18A campervan. Hence, the challenge of fitting everything I need into such a compact space led me to a brilliant solution: storing some items outside the van. This way, I can free up the living area for essentials and keep my journey as comfortable as possible. Read on for details of my DIY Promaster campervan cargo box installation on Thule rack that came with the van!

Why a Cargo Box on the Promaster campervan?

Initially, I considered cargo boxes specifically designed for the Ram Promaster. However, the prices were jaw-dropping, ranging from $1500 to $2500, and that didn’t even include installation costs. SO, determined to find a more budget-friendly option, I turned to Amazon and discovered a fantastic DIY campervan cargo box alternative for just $159.

Cargo box DIY
This was taken before I removed all the extra Thule parts. I wasn’t done, but I was so happy to see my plan coming together just as I had hoped!
DIY Promaster campervan cargo box installation on Thule rack
Finished Product. Love it!

My Amazon Find: The Aluminum Underbody Truck Box

After lots of searching, I eventually found this gem: Aluminum Underbody Truck Box 36 x 18 x 17 Inch with T-Handle Latch, Waterproof Truck Storage Organizer Chest. It weighs only 21 pounds, and since the Thule bike rack (which came already attached to the Promaster campervan) has a weight capacity of 77lbs, it was a perfect match. By mounting this cargo box to the bike rack, I could safely load it with up to 56 pounds of cargo.
Note: At the time I’m writing this blog post the box is sold out in the black color. I did find this listing, however, that appears to have the same product – just a different brand.

The Installation Process

The DIY Promaster campervan cargo box installation on Thule rack was a bit of a DIY adventure! Here’s a step-by-step breakdown of how I did it:

Modifying the Thule Bike Rack

I removed most of the Thule bike rack parts but kept the main attachment piece and the weight-bearing section.

Fitting the Box

To make the campervan cargo box sit flush on the rack, I had to cut holes to accommodate the 45-degree joints of the Thule bike rack. This was the toughest part. I used a hand saw, but if you have a power tool, it will make the job much easier. Remember to measure twice (or thirty times) and cut once!

Cutting the cargo box
Cutting the cargo box
Box cuts for the campervan installation
Cargo box hole cuts DIY
Finishing the Edges

The cut holes left sharp edges in the campervan cargo box, as you can see in the photo above, so I bought some rubber trim to cover the sharp edges. This not only improved the aesthetics but also added a layer of safety. It was very easy to install; just remove the red strip to reveal the adhesive, put it in place, press to adhere, and trim the excess with scissors. Easy peasy!

DIY Promaster campervan cargo box installation on Thule rack
Securing the Box

I marked the tips of some U-bolts with a paint marker and tapped them onto the box where they would secure around the bike rack poles. You can find the U-bolts I used here. I also used spare parts from the bike rack to prevent metal-on-metal contact.

Promaster campervan DIY cargo box rubber pieces
Adding Lights

I installed magnetic rechargeable motion-activated lights inside the the lip of the door on the box, so they illuminate the inside without blinding me. Two sets of these lights are in my van (four in total) because they’re incredibly useful. To kick them up a notch I added a thin strip of felt to the back of each light to protect the van’s paint when they’re attached to the exterior.

Awesome lights for campervan
Charging the Lights

I set up a dedicated charging spot for the lights inside the van, near the SkyBunk USB ports. This makes recharging them super convenient!

Love these lights
Now they won’t scratch the metal surfaces they stick to!
The Result

I’m thrilled with how much I can fit in the cargo box! Also, the addition of the lights was a game-changer, providing extra security at night. Aaaaaand, it’s waterproof and locks with a key, ensuring my items stay dry and secure!

cargo box light
Illuminated light
campervan cargo box
cargo box key

Final Touches

All that’s left is to critter-proof the holes I cut using spray foam and drill two more holes in the bottom. This will allow me to feed the Thule bolts through and stabilize the cargo box further, taking some stress off the U-bolts when driving on steep inclines.


Overall, I’m extremely happy with my DIY Promaster campervan cargo box installation on Thule rack that came with the van! Not only did I save thousands of dollars, but I also have a great sense of accomplishment from doing the work myself. I think it looks nice, and I love that it’s waterproof, lockable, and only sticks out about 22″ from the back door of the van, so no overhang flags required. I highly recommend considering a similar setup if you’re looking to add space but don’t mind a little DIY project to save yourself loads of money!

Happy travels!

Feel free to drop any questions or share your own space-saving hacks in the comments below!

Affiliate Links:

Aluminum Underbody Truck Box
(or THIS LINK if it’s out of stock there)


Rubber Trim

Magnetic Rechargeable Motion-Activated Lights

Felt Strips

Disclaimer: This DIY project is not endorsed by Thor, Thule, Promaster, or any of the product brands I used. If you choose to perform this modification, do so at your own risk.

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