Summary

This was inspired by lens42's macro lens mount on Shapeways. The designer of that mount tracked down a cheap ($4) lens from surplusshed.com that is suitable for iPhone macro photography. I wanted a mount that would slip over a case, and that I could print myself.
This version takes the mount design from my loupe mount and adapts it to hold a bare lens.

Update: Added 12mm version for Surplus Shed lens L4581, with a higher magnification.

Instructions

The inner diameter of the lens hole is 12.3mm, which on my printer holds the lens snugly without glue. If you wish to adjust any dimensions, download the rsdoc file and use DesignSpark Mechanical (free) to push the hole to any dimension you want.
The outer ring has a retaining lip, so you must insert the lens from the inside of the mount. It is easiest to drop it in from the side, using a pencil eraser to float it horizontally until it settles flat, then push it in from the hole on the mount side. The L4471 lens is a plano-convex lens; the flat side should face towards the phone.
There is an assembled STL (which is taking its sweet time rendering), but that is mainly to show how it looks when complete. You could print it whole, but you would end up with a rough bridged surface that might scuff a leather case. Better is to print the two separate parts to get smooth case-side surfaces, then glue or solvent bond them together, using the tabs and holes to aid in alignment.
Designed to be printed at .20 mm layer height and .40mm extrusion width.

iPhone 5s macro lens mount