I use my Garmin Etrex Legend on my T-boss. I bought the Garmin bike mount for it and mounted it to the grabhandle on the passengers side. Works great! As far as shaking the GPS to death, I don't think you have anything to worry about. There are no moving parts in the GPS to rattle loose. Mine reads fine and doesn't have any prooblems keeping up to date. (Unless I get into the thick woods.) On a short note, I also mounted a camera mount and use a cheap digital video recorder. That too doesn't get bounced around too bad.
It's all just nuts and bolts.