I went to Blancher Canvas on Rte 28 in Dunellen to get my dump trailer done. Granted, it isn't a metal box, it's heavy duty mesh netting. But they custom make a frame for the body then custom make a tarp with grommets and tie downs all over it. They even custom made the opening where we will hang our debris loader. Darn thing holds near 15 cubic yards now.
Some benefits...
Lightweight, built of aluminum polls and a mesh tarp so adds only ~100#'s in weight. Easily broken down/installed, just a matter of attaching aluminum poles (without tools) and throwing tarp over aluminum. Then, attach with tie-downs to trailer body. Can dump the trailer without removing canvas top because back has a flap. IMO, it was well worth the cost of $525 considering what I would have spent on metal or wood and the cost of converting the trailer from debris loader to material hauler.
Don't know if it will help you but I figured I'd chime in.