They are very simple to make. Just take an unpowered Ross switch and a piece of music wire. Bend the wire so it so the end can go through the throw bar, and anchor the other end 4-6 inches away.
The trick is to get the proper tension. Do this by placing a fulcrum, in the form of a nail or screw somewhere along the length of the wire. The points should easily be pushed aside by the wheels, but also snap back into the closed position.
