Summary

This is a derivative of the OpenJSCad iPhone 5 dock, with a slightly larger tab at the back, a gap for the home button, and a completely enclosed channel for the lightning cable.

Instructions

To create the STL file, launch Google Chrome or Firefox and go to:
joostn.github.com/OpenJsCad/processfile.html

Then drag&drop the .jscad file into the box.

The jscad has configurable parameters for:

margin around the iphone (for if it is in a case)
margin around the dock connector: 0.5 mm gives a tight fit in my case (RepRap Prusa at 0.1mm layer height)
to add a mouse ear at one end, to prevent warping

You can change these settings in the browser, press Update and then Get STL to get your customised STL file.

Parametric iPhone 5 Dock