Paul,

Not sure if it's any good to you, but I hired a TomTom Rider V2 for my trip last month to France on the 748R.

The brackets that came with it are obviously more designed for trailee style bikes and were useless for a sports bike so I just got two strips of thin metal about 20mm wide and 100mm long. I drilled holes in these that lined up with the holes on the back of the TomTom body and used the TomTom screws to secure the strips.

The whole assembly was zip-tied to the top yoke and worked a treat. I just placed some padding under the strips to get the viewing angle right and stop the metal scuffing the yoke itself.

Worked an absolute treat with no annoying vibration.

All in all it must have cost me about 30 mins of time as I had the tin lying around.

I'm sure you could do similar with the Garmin.

Guy