I have a Garmin 60CS with a bike mount, works like a charm and water proof to JS7 standards I think? Anyways waterproof enough to be submerged a length of time and still work. Not going to happen on the bike.
It was installed as below because I had stock riser, new riser allows it to be just left of center.

Here it is as mounted now.

I've left mine on for the most part of 25K miles, I hook my helmet on it to cover it while away because of what BigBear says, If they see it, it's tempting. Mine's not locked. Comes off quick and easy. But never comes out of this bike mount. They make a newer version of this called a garmin 60CSX and I highly suggest it for a Bike. Not too big, not too small, waterproof. Buy extra stuff and it goes into or onto anything I used mine on Mountain bike, Car, Boat, and Road*.