This modification allows you to thread though a 25mm wide elastic (textile) strap and create a loop with it for  your finger. 

I have had this on my phone for years and I really like it. if it is not in use, it is very flat

Anyway: 

• you will need to source that strap (approx 25mm wide, max 1mm thick)
• you will need to sew the strap together on the backside (inside)

If you have TPU and feel like modeling, you can also print that strap in TPU. Feel free to remix my Model from here: CMF P2P there is a suppressed sketch named “Inlay for TPU-Printed Elastic” which is drawing a insert that allows you to print the parts separately and in more print-favorable orientations. There is still some way to go in your remix, this is most likely a non self explanatory sketch so you will need to work something out :)