I have TT Zumo mounts for three bikes. The trail oriented bike I converted the TT mount to use the Montana AMPS mount.
1) Remove the Garmin Zumo cradle from the TT mount
2) Remove the sheet metal that held the Garmin Cradle
3) Use removed sheet metal as a form to bend an thin metal plate
4) Use removed sheet metal as an outline to mark the newly formed plate
5) Cut plate
6) Depending on how you mounted your Zumo previously you may have room to mount the Montana horizontally or vertically or both
7) Use the AMPS cradle to mark mounting holes in the new plate
8) Drill holes
9) Mount AMPS cradle to new plate
10) Put new plate back on TT mount
May look like this
I plan on using the AMPS locking screw to give a bit more security.