I had my son use a black turtleneck sweater to hide his neck and a pair of black sweat pants or joggers. He then applied the knee pads, elbow pads and shin guards. Once those were on he put on the suit, I cinched down the web belt and he completed the ensemble with the purchased helmet and neoprene mask. We added a noisy battery operated laser pistol and a reflective belt for good measure. All set to trick or treat in a one of a kind, generic, reusable costume for about $10. The helmet was the largest expense.
He was one happy kid!