Problem: The on off button on my handheld Garmin Etrex20 gradually failed over time, becoming more difficult to apply enough pressure to it to get it to switch on with my finger. I then tried very carefully using a pen to press on the button, but eventually the waterproof gasket developed a small hole. Garmin refused to fix it at my expense, even though the on/off button and the gasket looks to be exactly the same as in the current model. Instead they offered me a small discount on replacing the entire GPS unit with a new one. As I don't walk too many times a year, that would have been an expensive fix.
Solution: I cut through the waterproof gasket to properly examine the on/off switch. The switch is on the circuit board and it still worked. It merely needed something long enough and of the correct diameter to press on it. So I went to Jaycar and jury rigged a way to physically press the circuit board contact switch on and off, using a couple of small components. I then covered the new arrangement with some Gorilla tape and tested it several times.
Whilst the GPS is now no longer waterproof, it otherwise works just fine. Total cost, just a couple of bucks from Jaycar and the tape that I already had.
To keep it dry out bush, I just keep the whole GPS in a small freezer bag while I am walking and don't expose it to the rain. I should get several more years use out of it and when I do eventually choose to replace it (probably when my PLB battery next needs replacing) i will probably get one product that combines GPS location and emergency contact/signalling and 2 ways comms instead.