I’ve always been a fan of drawer systems. They help keep all your stuff organized whether it be gear for hiking, camping, tools for work, etc. Not only all of that but you can keep the top clean and pop a mattress in there and you have a pretty comfy situation there!
I used 3/4 inch oak cabinet grade plywood for most of it. I used square stock aluminum with skateboard bearings for drawer slides. This is a pretty popular set up for home made slides as it is way cheaper than purchasing 6 foot 200+ lb drawer slides. The drawers turned out pretty well. I also had to put extra seals around the tail gate and topper window. That also turned out pretty well.