Similar to a few other iPhone tripod mount options, my version features a hexagonal hole in the bottom, where an 11mm 20 TPI nut can be wedged/glued. The fit is pretty snug, so you may have to file it slightly to get the bolt to fit easily.
The mount also has a cutout on the bottom to use it as a viewing stand.
The design was primarily made with printability in mind, as I had some issues printing tripod mounts with a gorillapod-stye mount.
Sliced in Slic3r. 0.2 mm layer height, 15% infill, no support. Printed in PLA on Printrbot Simple Metal.