If you want to build a camper box on the FMTV frame, there's an old oil-field trick that might be useful. Build some long 'air bags' out of 6" fire hose. Just flatten the ends, drill holes through some flat metal stock, say 1" x 1/4", holes about every inch, and use two of those to sandwich and seal the ends of the flattened hose. Use some sealant. Drill a hole in the hose and put in a Schrader valve in a convenient place, the kind with clamp nut. BTW, figure out where you want the Schrader valve and put it in first.
A pair of those along the edges of the camper would shock absorb it. Tie down the load with short straps and big footman loops and tension the air bags against them.
