TechMatte MagGrip Air Vent Magnetic Universal Car Mount mounts on the car air vent and uses a magnet that's being attached to the phone (can be placed inside the phone's case in most cases).
Con Requires a magnet unit for each phone
Each phone that needs to be mounted needs to have a magnet unit. If you don't use the adhesive to attach it to the phone then you can transfer the magnet between phones but that could be annoying. On the other hand, you can get more magnets and it's pretty cheap.