I was thinking something along using two strong elastic bands. Like the really strong ones you get from holding the broccoli (western broc) together and the ones they use to keep the claws from pinching you on the lobsters.

I'll have to draw out what I'm thinking unless you can some what figure out what I'm thinking right now. I was thinking of stem mounting that GPS. Say use two really small PVC tiny pipes of about 1 inch long for the small piece and 4 inches long for the other piece. With the small piece you drill a hole on the sides large enough to run a ziptie through it. THat is the piece you'll be zip typing to the stem. The second piece you screw it togheter with a screw to the first piece before stem mounting it. Now add some gun tape or foam and tape up the longer piece so it can give a soft buffer cushion for the GPS then with two strong elastic bands you put that on the top of the GPS and the bottom of the GPS allowing all functions to be still operational.