I agree that the best answer lies in using a thicker foam to begin with. I've also heard that the foam can be strengthened by applying a top coat of something called adhesive ground coat, a thin cement type product, but I've yet to be able to find it. Neither idea is much help to you at the moment though. Zombies idea seems to have the most merit. Reinforce them from within. Or maybe some chicken wire glued to the back, and then covered with monster mud.