I used to fly at a club that had some nasty thicket around a good part of the field that ate airplanes on a regular basis. I went to radio shack and built a buzzer from some cheap parts there. I used a spring loaded switch in a "deadman" arragement that was pushed closed by a wheel collar on the throttle linkage. As long as the switch was closed, the buzzer was off. It had it's own AA battery which was taped to the buzzer and switch. Basically, if the throttle was reduced to idle the switch opened and the buzzer started. In the even that the fuse broke up, the odds were that the whole buzzer assembly would eject together and buzz away. I never tested the total ejection senerio outside of the workshop though. The buzzer did help me locate my plane a couple of times though.