Was toying with the idea of shifter knobs myself. My idea was trying to figure out a way to make a clear one out of a resin with one of the VX HotWheels encased in it (silver in my case).
Sgt Batguano has a point in that the '99 wheels have a different kind of center cap with three screws that go into the wheel between the lug nuts. I'm not sure if the areas where these discs would go are the same size on the 18's and 16's or not.