I bought these mag holders, only used one with the 17mm to 17mm ball to ball adaptor (2nd link) with an existing clip mount that was 17mm. I did not want to put a metal backing on the phone because of wireless charging. and like all these magnet suggestions stick it to the fabric with metal behind it to the side of the screen...
https://www.amazon.com/Magnetic-Upgraded-Stronger-Magnets-Samsung/dp/B0859VRH6C
https://www.amazon.com/Composite-Industry-Standard-mounting-Brackets/dp/B085VQTX2Z
