[KeyserSoze]

First of all this is a great idea, I always thought it would be cool to do R/C rally, but with cars not ugly dune buggy looking cars.

You could make the bumps and stuff with cardboard and then put the putty stuff over the cardboard. It would probably get messed up if you put much weight on it, a bit char'g would be fine though. You could form the bumps out of hard styrofoam (like bike helmets) and then put your putty stuff over that. these options would also make your track lighter than if u were to make the jumps entirely out of the putty.