What Iv,e done 3x now is to buy a flatbed with stake pockets. Then insert c channel in the pockets up the the height you want the body to be. Flat steel up under that, with square tube at the top. On top flat steel across, square tube across above that for strength. Water runs to the sides on top, inside is smooth, dumps nice, is rigid. and works very well. My first two designs were slightly different but the last was the simplest, looked and functioned best too. All welded together with my little 110 wire core mig. Easy, simple and worked well for 5 years now. 1/8 steel sheet, and square tubing, c-channel to fit the pockets. Painted with brush and roller, good primer, then black rustoleum. Tailgate barn doors, trickiest part is getting the geometry right so they close but also fold flat against the sides. Made the hinges from bolts and large steel bushings. Old Lumber trucks work well, hard to find ones with pintle hitches though.
Mike