I was trying to see if I could mix Flex and PETG in an MMU2S print, and after a successful test print I decided to try a mixed-material iPhone case. 

I am really happy with how it came out. 

Works with an iPhone XR. I used Tecbears / Sunlu Flex 95A and also Tecbears / Sunlu PETG blue filament. I've provided the complete settings I used as a PrusaSlicer project. The print would probably work fine as TPU only as well, though I like how the more rigid back feels. 

There are two small pillars on the bottom port - around the Lightning jack - that will fall out when you try to use it. Originally I was thinking that I wanted the solid plastic there but it turns out that the case works just fine without them. So I shrank them to 1mm and they're effectively supports now. I suggest simply removing them after the print is complete. 

I had to clean up the Flex a little bit with heat due to stringing, but it fit great, feels good (except the bit I cooked and isn't quite flat!) and looks good after that. 

v3 has a little bit more TPU on the sides and should be more durable.