iPhone6 Case shell model from Thingiverse.com and then used Autodesk 123D Designer to customize it by stamping out my son's initials into the back. Doing this was rather simple - by using the "Combine" / "Subtract" feature of 123D Designer - using the case as the target model, and extruded text letters as the source (the "stamp").
See what I did wrong in the picture? I stamped the letters from the INSIDE of the case, making them appear backwards from the outside of the case. Lesson learned from failure!
The walls of the case looked rather thin, but the case did fit his iPhone for the most part.
However, there was some play in the length - where the iPhone moved up and down within the case by approximately 1-2 mm - it was noticeable. The second problem came about on day 3 or so - when my son dropped his phone in a rather un-eventful way (no threat to breaking the phone itself) and the case broke. One of the sides cracked off. My next attempt, I might see if I can re-inforce the model by filling in the small gaps where the sides curve along the case walls and connect to the back.
Some Print Details:Filament: JET PLA, Orange, 2.75mm (measured, although labelled as 3mm of course)
Model: iPhone6 Basic case
Time to Print: 83 minutes