Not Long ago I broke my headlight by dropping it, and Had to get another. The model I have only comes at a set with the tail light, so I wound up with a spare.
This model tail light's mounting bracket is a nearly perfect fit for the Garmin Etrex GPS.
https://buy.garmin.com/shop/shop.do?pID=6403
There are 2 small plastic stop points that need to be shaved out to make the etrex slide onto the mount all the way.
I don't have it 'lock' on yet. I hadn't quite worked out the best way to keep it from coming out. It slides onto the bracket from the front, and fits snugly.
The Tail Light on the mount
The mount only, on the handlebar
The Etrex GPS mounted