Jay, Nikita, and I made a rubber band car and it rolls fast. We put training wheels on it to keep it balanced. We also used CDs, skewers, faucet washers, putty, duct tape, and a rubber band. We used a 5 to 6 inch piece of cardboard.
Our car is exactly 5 inches long, and we cut two inches off in the middle so there would be a little hole. We used faucet washers for training wheels, so that the car would not crash into the wall. The training wheels were very little, but very good at keeping it stable.