Have been really busy with life, but I did just complete a direct to post mount adapter for the Garmin RCT715 and the Trek Madone! Here are a couple of images that I received permission to post
https://www.shapeways.com/product/6X2SB52AV
No clunky o-rings, no rubber spacers, just a solid direct to post mount.
Note: Personally I would not run the Garmin camera upside down as it does not have auto-orientation, but it still fits.