I secure my GPS III to the top of my Kneeboard. It's an ordinary kneeboard (Airtour CB3 Control Board) half A4 size.
I use a simple DIY right angled mending bracket which is a strip of metal 6 inches long and folded in the middle to make a right angle. They are used for mending corners of furniture etc and have pre-drilled screw holes.
One end of this is pushed in bettween the back of the spring clip and the board itself. The other end which is now sticking vertically upright from the board is used to mount the GPS using the screw fixing on the back of the unit. This can be bent to the right angle for the best screen view.
Its a simple no glue, instantly detachable method, but needs a remote aerial. No messing with dash mounts or velcro. Holds the GPS very firmly, above & out of the way of the flight plog.