I've done the bumpers-as-structure thing before, and heartily recommend it. Use good, strong hardwood plywood (finish grade), and enlist the help of someone with quality woodworking tools (or a milling machine, if you're so inclined), so that it's exactly the size you want. The idea that the frame must provide
substantial structural support for the bumpers (rather than the other way around) is nothing but a silly conceit—any support whatsoever satisfies the requirement (R26). But whatever you do, don't make your bumpers difficult to remove as a result—ideally,
each segment should be removable by a single person in 10 s without tools.
Presuming the 2015 rules remain similar to 2014, there is very little guidance on what constitutes the part of the mounting system that forms part of the bumper, in contrast to the fairly specific (but slightly geometrically-challenged) frame perimeter rule. You can use that to your advantage, while still complying with the rule.
But you shouldn't let something that is clearly part of the rest of the robot overlap the plywood of your bumpers—that's asking for trouble. Under some years' interpretations of the bumper rules then in force, that's occasionally and inconsistently been ruled legal, but it's a bad idea to attempt it.