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).
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For around $10 this is probably one of the cheapest mounts out there.
It works as long as you attach the magnet to your phone which could be just putting it between your phone and its case.
Pro Powerful magnet
The magnet is powerful enough to hold your phone even if the 2 magnets are not 100% aligned and touching.
It's really easy to install and uninstall the mount. Just push into vent to install and pull out of vent to remove.
Con Blocks 1 of the car's air vents
If you're using the AC on a hot day it could be annoying. Using the heat on a cold day does the same and can also heat up your phone, depending how hot you set your heat to.
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.