If you have any replacement erasers for a mechanical pencil around, use a razor blade to slice it into 3/4 discs, poke holes through the middle with a needle to make them donuts, then thread them onto your cable. Of course that requires taking the cable out of the brake, but you could also try slicing radially to the center hole, putting them on with slice away from the frame, and see whether they stay on or not.

You could also fashion bumpers from Baby-Bel cheese wax, if red matches your color scheme better. Or chewing gum.