A simple hook that can contain a magnet up to 22.5mm x 18.5mm.

There are two versions. One is open so you can glue in a magnet after printing. The other is closed so you need to pause your print and add the magnet right before the layer that will cover it. The closed version can take a magnet up to 2.5mm thick, while the open one can pretty much handle one as thick as you like. The open one tends to stick better, since the magnet can be right against the surface, but I like the closed one as I feel it's less likely to cause scratches.

I used a magnet from an old hard drive, which is why the dimensions are what they are. I use the hook to hang an oven mitt on the fridge.

The closed version is designed to be printed at a .32mm layer height, with which it will have just two layers below the magnet. For either version, I suggest printing at a thick layer height and high infill for strength.