I cut a small rectangular hole through the balsa either above or below the switch on the side opposite the muffler. The hole is the same size as the voltwatch LED strip. I secure the voltwatch to the inside of the fuselage with the LED's showing through the hole using about a 3 inch length of Blenderm tape. I put a little bit of white (or matching color) fingernail polish on the balsa then put a small strip of clear monocoat over the hole.

It is fuel proof, and very visible when I want to look at the voltwatch, which is just before and just after each flight.

CGr.