A view of the inside of the chuck wagon shell:
And here's a concept drawing of what the chuck wagon compartments might look like.
The rear two compartments are boxes that attach to the inside walls. They'd have shelves in them, and perhaps an elastic cargo net, to keep things in the shelves. They could possible also have a Molle grid on the back to hang pouches and other Molle items. More concept views to come of what the inside of the boxes might look like.
The depth of the boxes would probably be the same as the depth of the part of the tub that angles out; or put another way, the back of the boxes would be vertically in line with the side of the tub at it's narrow part.
The front compartment is a shelf that runs all the way across. There's a wall on the rear side of the shelf to keep things from falling off; the shelf runs all the way to the front wall of the chuck wagon shell.
All of these interior parts would be removable, so the chuck wagon could be used as just a large covered cap for the trailer if large items need to be hauled; also having them removable allows the chuck wagon to be disassembled into smaller pieces for storage when it's off the trailer.
Comments and suggestions on the compartment design are very welcome, I haven't completely finalized the design yet.