I have two barrel shaped Neodymium magnets glued & taped to my mobius for a few years now, 12 mm diameter 8 - 10 mm think.
And one of my rear cameras are held in place by a single even larger Neodymium magnet, this also work flawless for years.
Mounting off the metal frame of the rear hatch in my small 5 door hatchback, and off a piece of metal folded over so it slide over the headliner for my L side camera.
The magnets for me will soon no longer be needed as i will move away from using single cameras to dual channel systems, where it is more easy to retrieve the memory card ( storing footage from 2 cameras ) in 1 easy accessible location Vs having to go to the individual single cameras where they may be installed, which was made more easy with the magnets so i did not have to mess with the little memory cards in the car so just grab the whole camera instead.