Summary

There are plenty of mobile stands but I found none of them good enough, so I created this one.

The best is to print base in TPU. It will be flexible enough to adapt surface curvature. Print arm and plate in PET-G or ABS since it can be pretty hot in a car. Screws needed:

2x M3 Secure Nut
1x M3x23mm screw or longer for base-arm attachment
1x M3x19mm or longer for arm-plate attachment.
3x Neodymium magnet 20x5x1.5mm

You can customize the plate to fit your magnets. You can also fine tune how hard it will be to move arm or tilt plate by tightening the screws.