If you build your own box, look at the Westfalia utility trailer. I have one currently in pieces. This one's not mine:
It's 6mm plywood with a
welded riveted and galvanized angle-iron frame. You could laminate two 1/8" sheets of plywood top make curved sides.
The suspension is trailing arm with an elastomeric snubber. The problem with that is the pivot point has to be higher than the axle centerline. Maybe intead a Speedway dropped Fad T-bucket axle with disk brakes?