I used this specific magnet: https://www.amazon.com/gp/product/B097CGPZV3 but had to glue it on rather than use the included sticky pad.
I printed in PETG and have had no issues with it staying in the truck. I also used museum putty to keep the insert itself in the slot around the peg holes, as they were not snug enough by themselves when removing the phone.
Used: 20% infill, 4 walls, .15mm layer height.
The author remixed this model.
Added a slot to glue a Magsafe magnet.