Must be on a flatbed? Would it be OK to drill some holes for bolts? My racks for my F-350 pickup are 1x4s carrige bolted to some 2x4 uprights, contoured for the stake pockets. I bolt three 1x4's onto the ends of the sides across the front, and I use ratchet straps to hold the "tailboard" in place it's three 1x4 uprights and three 1x4 cross pieces.
The same would work for you, if you could bolt some brackets to your bed.