One of our track locals made a home-made one. It's pretty cool - it uses a clamp from a workstand to hold the bike by the seatpost. It's a manual-release via a string that releases the workstand clamp handle (thus releasing the bike).
It also is designed to tip over and be rolled off the track, which is nifty. IE, there are wheels on the face of the start gate that faces the infield. Once a rider is released, the operator tips the gate over, it lands on its wheels, and rolls off the track.