[RRLedford]

The requirement for 100% full, CONTINUOUS, intimate contact of bumper plywood with the entire perimeter of the frame is rather excessive. Especially when there is no similarly draconian requirement for exactly where within the 5" height of the plywood that the frame makes contact. If frame only touches at the top or bottom 1" of the plywood, the bending torque force with a 4" cantilever of unsupported plywood at middle of side is also very serious with dynamic impacts!
It would make much more sense to have a simple spec for a maximum allowed spacing distance between points of support of frame in contact with bumper's plywood. If this spacing distance was between 3-6" the chance of breaking plywood would be minimal.

We currently have three aluminum plates on all four of our bumpers, one at each end and a 3rd in the middle. They are mounted with flat head screws going both ways, outward into T-nuts on plywood and inward into T-nuts inside frame slots. Screw heads are all countersunk into the thickness of the aluminum plate. The plates also stick up higher than the top of the bumper so that we can access the mounting holes for the plate-to-frame screws.

We were assuming that these plates were part of the bumper, since they fit within the 1" max. thickness of plywood plus mount H/W, but now we have the issue of plates only giving intermittent contact with frame-to-plywood. We also have the issue of plates sticking up past plywood. Is this still legal, as long as the aluminum plates still remain within the 10"-16" above floor bumper zone?
Perhaps we should consider treating the plates as part of the frame, and then filling in the gaps between the plates with similar thickness aluminum bar along the frame face, to give a 5mm expanded frame perimeter. Fortunately, we are not at the max 28"x38" size already and have some room to do this.
-Dick Ledford