Starting from @yapiodesign 's CupFone Replacement Base I have created an adapter to 17mm ball joints.  It allows connecting various phone holders from China express including magnetic ones.

I have put a hole for a screw in the middle for added strength, not mandatory, you can try it without first and see if it holds.

Print in place with minimal supports.  Recommend at least PETG if not ABS due to this being exposed to potentially high temperatures in the car during summer.