The box is an Aluminess jerry can tray that I bought used from another member here......so I took the leftovers from the Chinese moto hauler and made the receiver mount for the box. It was crazy easy, kinda made for each other. It plugged into the receiver completely separate from the moto hauler.
That Aluminess tray has since morphed into a DIY swingarm box.
The supports are 1.5" x .125" wall I think..it's been a while. When I designed and fabricated my rear bumper, swing arms etc. I made longer supports out of .093" wall since I had to clear the storage box and spare on the back of the van and wanted to see if I could make the moto hauler lighter. This proved to be under designed after hitting an uneven road surface with the moto on the hauler and bending the supports.
The latest config uses the original .125" wall supports sleeved with some 1.25" material. This accomplishes 2 things; it spaces the rack further from the rear of the van so the bike can clear the box and tire and it doubles the wall thickness (sectional area) right where the supports go into the receivers, where they bend.
I have stops in the receivers so that I can slide in the carrier without having to align the holes to put in the clevis pins. When the carrier supports are bottomed against the stops the holes line up.
A bit heavier but pretty bulletproof at this point.