Here is a concept drive base/ bumper mount system that I designed a few months ago. It uses two plates, one on top, one on the bottom, and the bumpers are attached by a piece of 2x1 box tubing which is bolted between the plates. This probably isn't practical without significant prototyping, but I will put it out there anyway.
https://drive.google.com/folderview?id=0BxrM5TU8rVcOYnVNaEZYczlCcjQ&usp=sha ring