This mount replaces the two piece mount used on the stem phone mount:
http://www.amazon.com/Arkon-SR088-S-Flexible-Gooseneck-Satellite/dp/B000EHYAJ6

This is compatible with my iphone 5, iphone 6 and iphone 6 plus mounts:
http://www.thingiverse.com/thing:448391
http://www.thingiverse.com/thing:471668

The hole pattern is 30mm x 37mm and uses #6-32 screws and nuts.

Included Solidworks 2015 file and IGS file.