[IKE]

Cool Idea. If you want it to be able to be knocked around, but never flipped over, a low CG is good, but the way the upper structure works relative to the chassis is really what is important. For instance A sheet of plywood has an extremely low CG (when flat), but I can easily flip it over. The real trick to the Bop-a-clown is that the curvature of the inflatable makes it so the CG can never be above a tangential surface unless you flip it over perfectly balancing the weight on top.

If you took the Kitbot chassis and shortened it into a box (approx. 24x24"), strapped a trackball to the top of it, you would have a good initial prototype, and likely wouldn't need any special motors or pancake batteries.