I've made a plywood and 2x4 framed one for my one ton truck and 12k dump trailer. Lots of long screws with washers on the heads so they do not pull through with bouncing down the road. They have held up very well. I have loaded them full with chips, fire wood, and every size log/chunk of wood. I did box frame the roof every 4' as per the interior side stakes I think this helped a lot with torsional rigidity. It's solid and gets mistaken for metal. I coated the plywood with 2 coats of rustoleum prior to building. Going to throw another coat on the whole trailer and box now as the metal is starting to rust after only 3 years. Sorry no pics. Gotta get a real phone.
I think a full steel frame(sides tied into roof) would be great with plywood panels.
I call my current box a prototype for the metal framed one but it has proven to be quit durable so far. When I fab a metal one I'm thinking of making the top open down the middle lengthwise folding the two doors over the sides to allow top loading of what ever. A ladder and/or gear rack for the top or sides would be sweet.