I like the idea of a stainless box or at the least legs and the back-wall I would have to do some weight comparison I don't see much puncture resistance from a flimsy aluminum box. I know I know the load is never supposed to touch it , but if it did. My buddy of 35 years owns a small metal fabrication shop down here in Florida he lets me fiddle around in.Most of the metal he deals with is heavy stainless or aluminum. if I shear and brake the metal up he will just charge me for the Mexican welder and the metal. . He is always interested in seeing new stuff being built , he mostly does custom high work.
I’ve never done that particular job on this truck but I think there’s plenty of room. The evaporator and fan for my APU is mounted on the back of the rack so there’s at least 12” of space between the rack and sleeper. I can also sit Upright between the frame and driveshaft with ease on this truck, it’s a pain to get there but “roomy” once you get in position.
I ran a 5 door when I had my condo, after I chopped the top off I ran it as a 3 door. Now the rack is gone and I bought a new trailer. These 65 ft beams will ride nice hanging 5-7 feet off the front. I’ll never run a rack again.
Iconic Metal Gear makes some stout racks. I have the single roll up door, and i know they make a 3 door roll up.