I have taken a different approach... mounting the GPS to the quick release tank box I made.
Garmin Nuvi 5000 in a Aqua Box:
An aluminum plate I bend and drilled out holds the Aqua Box then covered with black gaffers tape.
Garmin Colorado 400t:
Garmin StreetPilot 2820 w/XM:
I've rotated through the StreetPilot, Nuvi, and Colorado and only the StreetPilot really gave me the options and control I need for my adventures... The Nuvi was the worst, gave me the most trouble, and the Colorado was OK, again just not flexible enough.
Tired of screwing around with consumer GPS's a Garmin GPSMAP 478 is getting installed this week using the marine mount on my tank box. Pics to follow.
Here's the version 2 tank box DIY thread:
Version 1 tank box thread with more quick release mount info: