Print with backside down TPU at 120 or 200um layer height. The infill can vary depending on how flexible you want the case to be. Ex. 20% is very flexible and easy to insert, and 100% is much stiffer.

Overture TPU worked well. Print on a textured sheet or blue painter's tape for a nice grippy texture on the backside.

Phone should slide right in if you've printed it correctly:

Original post on Thingiverse is linked for reference.

https://www.thingiverse.com/thing:4406119