I am currently building a body for one of our loaders, I bought a used compactor box from a local dumpster company,( its the kind you would see behind retail stores etc for compacting cardboard etc) the top of the box was pitted with rust and they couldn't use it as a compactor anymore. I cut the top out of it, cut out the hook system from the front and am patching that area in with new cross members and plate, we cut the rollers and rails off the bottom, and are notching the top and using 4 inch box for the top rail, the box is ribbed pretty well because it was meant to hold the pressure of compacting the load. the door end can either be patched in at the opening or what I am doing is installing 4 inch angle iron vertically on both sides to form a channel that we are going to slide 4x6s made into panels to close in the rear, but the can be removed if you have to clean the body out.( We have worked some fema clean up contracts that required the bodies to be swept clean between loads because they were paying by weight, so I have to have an option to be able to fully clean out the body.)