I took the existing v2 design by Bronan (which he borrowed from DennisP's MagSafe car mount) and made the shaft larger to withstand more abuse (as this car is a CarNextDoor / Uber Carshare loan vehicle).
I printed flat on its base, support required under the ball.
Installation instructions:
Note: this was for my RHD Mazda2, so flip this if you live elsewhere!
The author remixed this model.