There are a few iterations of this but they many of them use weaker magnets and just rest on top of the corner screw heads. This allows the top panel to vibrate and creates a poor seal if you're trying to keep it heated. 

This design uses larger cube magnets that make direct contact with the screw head while allowing the TPU ring to push down firmly against the panel while capturing the screw head itself. The hole in the bottom is snug on purpose, once you give it a slight twist and feel it seat they are really hard to move off and will resist getting knocked about. 

Magnets: https://www.kjmagnetics.com/b444-neodymium-block-magnet