My friends has his mounted on the tung like in post 6 but with no box behind it so it's free standing with no problems in a year and half. It's usually loaded with 3 full jugs at the start of a trip, if not We just keep the fuel centered on the rack like in post 6. (1) full jug put in the center, (2) full jugs put one on each end, (3) jugs, one full with two half full, put full jug in center position with half full jugs on the ends. Center the weight is the idea.
I mounted my rack to the top of my steel tool box lid, my trailer is small with not much extra tung, I have the tool box and a folding wheel on the short tung already