I agree with PenMan. The installation was beautiful no doubt, and the photos were great -- kudos to the OP. I like how the bed neatly clears the top of the tool box. But I hate to throw cold water on this, and am only saying this in the interest of safety.
I too am an engineer (still practising) and like PenMan, I would not follow this trailer down the road - period! The toolbox frame as well as the spare tire rack are both trying to pry the bumper off its longitudinal supports, and typically, these hybrids don't have significant frame-support to their bumpers -- that is the weak point. Even then, the bumpers are thin-walled tube so the welds to the bumpers are small. Last, the load is not static but dynamic so the issue would be low-cycle fatigue overstressing these light welds, which would most likely not be devoid of stress raisers.