Figured i would share my rear mounting solutions for anyone else looking and having trouble finding some. For the single I initially mounted to the saddle rails using a k-edge clone mount from Deal Extreme (no longer in stock) but did not like the loss of a saddle bag. Ultimately I mounted to the seatpost using a ebay clone gopro handlebar mount ($3 shipped from china), the garmin "industry standard" adapter, A 90 adapter and a standard adapter



While the handlebar mount is oriented 90 degrees off of ideal this actually ended up working to my advantage since the indexed garmin pieces are tight, the camera does not fall. While even tightened pretty darn hard there is still some side to side movement possible in the GoPro pieces.

For the Tandem I used the Garmin 1/4-20 adapter