What is a legal bumper?

[lemon1324]

Our team has mounted the axles above the kitbot chassis, and the rail is now 2.5" off of the ground. As the bumper zone starts here, we cannot bolt the bumpers directly to the rail. We were thinking of joining the bumpers to each other at each corner to get a rectangular box of joined bumpers. Where exactly is the plywood backing permitted to go in this scenario? (Alternatively, a clarification on the interpretation of "corner" from rule <R08> would help)

Thanks