The entire thing (minus the lid) is made out of one partial sheet of 1/4″ OSB (leftover from another project), then painted with the same epoxy-coat I have mentioned before. That will give it a protective coating on top that should be difficult to scratch or damage by loose items bouncing around.
These pictures are of building the main box, before adding dividers or a top.
Now come the dividers – I used more of the same sheet of OSB, cut to size and screwed into place. I attached them like this because it’s easier, but the dividers could be made adjustable if someone wanted to spend the extra time doing that. Also, these dividers were spaced randomly so each compartment is a different size. If there’s something specific you need to store in here, keep that in mind.
The only thing left is the top. Again, keeping in mind what you wish to store, this too will need cut to size. Because of the items I will have in/on it, I opted to have a split lid to make access easier. This is a particle board shelf that just happened to be the right size to fit on top, cut into three pieces and laying on top of the box in the position it will be attached.
Cabinet door hinges work great for this, and it happens that I had several left over from a previous job.
Finally, everything painted, put together, and installed:
One last thing to keep in mind – Make sure that this will not block access to your spare tire, or if it does, build it in such a way it can be easily removed. Changing a tire in the dark on the side of an interstate is no fun, and even less fun if you have to empty your entire trunk to get to the spare.