We have a whiteboard attached to our fridge. Unfortunately, built in magnets in our markers are way to weak. Anytime the fridge door were closed they would fall. To solve this problem I created a super simple magnet holder with built in pocket for a very strong neodymium 32x5 mm magnet. This holder can be used for regular whiteboard too.

Print with 0.2 mm layer height. No supports. Pause print at layer 440 (88mm) to insert the magnet so it ca be securely built in.

I've attached 3mf file with sliced model and color change to insert the magnet.

Magnets that I had lying around: https://www.amazon.com/gp/product/B07FL7NCYN/