After printing Roby81's model, I found that there was too much play around the edges and it wasn't deep enough for my project. I needed something that would hold the screen snugly and be deep enough to be almost flush with the front of the screen.

The inner dimensions for this mount are 55.6mm x 85.6mm x 5.9mm which will hold a Kuman 3.5" LCD display firmly in place without the need for screws.

Kuman 3.5" TFT Display for the Raspberry Pi 3B+