This design was used by my wife at the hospital during COVID. They would go for lunch and needed a clean way to store their face masks. They had metal lockers to this allowed them to put their masks on the hook and retrieve them after their break. I printed a number of these for the staff but these can be used for anything that is not two heavy. I use them on our fridge for decorations. 

Being that the back is made of plastic they can slide fairly easily but face masks tend to be very light. If needed a very thin layer of rubber or sticky paint can be added to prevent sliding but we never needed it. 

I printed is sitting upright with-out supports. Magnets are from harbor freight and are 8mm by about 3mm. They just press in tightly. Cover also is just pressed in. Back wall is 0.8mm so you can print it with up to a 0.8mm nozzle which is what I did.

Happy printing.