I use a 'Pilot 111' in the Chippy, velcrod onto the RH side of the coaming using a small wooden 'former', shaped underneath to fit the curve of the coaming, and flat on top for the Garmin mount.
It works a treat. Very useful for groundspeed and reporting point estimates, and as a back-up confirmation of position. I only once saw it fail - in an AA5 over the Lake District. We had 2 'Pilot 111's on board, and both went 'zero signal' from Morcambe Bay to the Solway Firth - so don't rely on it!
SSD