Gobo Holder

Also called a "pattern holder" or "template holder" (and sometimes by the slang term "dipstick"), a metal frame with a handle designed to hold metal discs (gobos) that are inserted into an ellipsoidal reflector spotlight. A gobo holder fits in a specially designed slot in the instrument located between the reflector and lens assembly. The most common gobo holder is (essentially) two metal rectangles with a round hole in the middle that sandwiches the gobo between the two pieces of metal to keep it in place.

Available from most fixture manufacturers, and City Theatrical (pictured below), Apollo, and Rosco.