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JonnyHK, Wow, it looks like you model it in foam first? I wish I had that kind of talent. I do have the quarter turn Garmin mount (male portion) up on Thingiverse, It was up there in STL and STEP, but I just added the Fusion 360 file, If you need it feel free to download it from here:
Garmin Quarter Turn Mount Plate by WheresWaldo - Thingiverse
Clarification question: Is this a friction fit of some kind or does it clip in place?