This is a simple passive kill switch that can be constructed in a tiny box like the 1590A or even smaller. The circuit is symmetrical so either jack can be the input or output. The resistors were added to make sure that the output of the circuit driving the switch was not shorted to ground, which might cause a problem with some designs. Use a normally-open momentary switch.
A 5k potentiometer can be added so that the amount of signal reduction can be adjusted. In the first version, the signal will be cut completely off when the switch is closed but the version with the pot can be adjusted so that some signal still passes through when the switch is hit.
The circuit is non-critical but a shielded box is still recommended.