Ok, something I have experience with. A friend of min built a ROV for underwater filming, but required battery operated lasers for sizing of underwater items. We built small battery powered lasers into housings, but have a difficult time finding a waterproof switch that would handle any sort of depth. o what we did was place a reed relay on the interior of the body, with a magnet affixed to te outside. Once the magnet was removed, the lasers lit up. You could probibly make a thumbswitch sort of device to have the magnet move from one position to another on the outside of you casing, triggering a reed like we did.