To follow up, here’s a few photos of my setup. Seems to be working very well. I haven’t driven over speed bumps at highway speeds but it sure seems stable. Magnets are strong. It breaks free easily when pulled but the force to make it come loose seems beyond what’s going to happen in the car.
One note: the larger platform of something like the belkin vent mount would be useful here. The round holder I printed for the MagSafe charger is barely larger than the charger. If it was squared and went a little higher up or down it would prevent more lateral movements that could cause the magnet to break free. I do not believe there will ever be enough force, barring collision, to have enough force to pull straight off.
I need to learn to make my own STLs heh.