We were planning on doing something similar with our bumpers, and having seen this we're carefully considering the rules again, which led to a few questions from our handy local inspector
First, a picture of our idea, as accurate as we can be with pictures is attached, showing both a side view and a top view. now, a description:
We use C channel that is slightly larger than the kit-bot frame on the bumpers. It's attached to the bumpers securely (most likely with two bolts/tee nuts per section of C channel). When we attach it to the robot, the C channel fits snugly around the frame, with holes for bolts to drop through vertically to secure it. Due to the snug fit and the bolts holding it in place, the bumpers are securely mounted, with no wiggle room - i believe this would meet <R07> I.
The C channel would be Aluminum, less than 1/4 inch thick. Since this is snug against the frame (with holes drilled out of the frame to allow incursion of the bolt heads holding the C channel to the bumpers), the total depth of hard materials would be less than 1 inch (3/4 of plywood and, for example, 1/8 of aluminum). This would meet <R07> N.
However, we aren't sure about <R07> M - specifically the statement that the bumper has to be supported along its full length. Our robot design would make it difficult to have C channel along the full length of the bumpers - most likely we would have 2-3 sections maybe 3-5 inches long. Would the resultant 1/8 inch gap between the plywood and the frame, in sections where there's no additional C channel, be permitted? A simple solution to this, of course, is to build out the frame in those "unsupported" sections an additional 1/8 inch, which would provide a flush mount for the bumper (pictured in the attached image as red). Our feeling is that 1/8 inch is well with in the designed flexibility for plywood of this type, and such a gap would pose no structural hazard to the frame or bumpers. In fact, you might end up with a gap that big from simple warping of the plywood.
We ask this with the full understanding that you aren't the GDC, and if asked to the GDC we would get the canned "we cannot comment on specific designs..." response
Since at least one other team is considering this, we figured posting our concerns and possible work around to remain within the rules could help the community.