I used a cheap snare from a snare drum (ebay I think) cut in two. I screwed it onto a dowel that can be turned. The square, transverse piece is a stop for the snare when it is turned away from the tapa. It has the dual purpose of silencing the snare when it is not in use, as well as keeping the snare from moving too far away. I've used two different controls on the outside. On one cajon I used a dial with a lock-screw through it. The lock-screw presses against a large washer under the dial. The other cajon has a bicycle wheel quick release. As you can see, there is only one (½) snare in one side, giving different sounds to the two corners of the tapa.
Jens