I tried several cases for the iPhone 5 and finaly came back to the Customizable iPhone Case by makerBot. It´s simple and it simply fits. But because of the edges it was not so handy. So I made the frame round. And as I wanted to connect my magnetic lenses, a altered the camera hole. At last I added a masonic theme to the back by cutting 0.2 mm out. This came out great.

I will give a blank back and the FreeCAD files as well to you, so you might change it. Have fun.