He uses a little CNC mill to create wood mold masters for silicone molds which he casts polyurethane parts in. It seems to work well.
"Both the original Omnibot and this revised approach feature a novel reconfigurable drivertrain, which enables the robot to perform on-the-spot 360° turns and to effortlessly alternate between forward and sideways motion. As opposed to most other omnidirectional designs, Omnibot can do so without losing registration with the environment, and without relying on exotic and expensive components. In fact, it uses just three extremely cheap brushed motors and four regular wheels."