Full disclosure, I took my inspiration from this design: https://www.thingiverse.com/thing:1044898/files
It was too big for my needs, and didn't fit my magnets, so I started from scratch and designed something like it that worked for me.

Magnets needed: 4x - 6mm x 2.75mm

This is a quick-release magnetic keychain that can be used for anything you need to quickly/regularly release from the main keychain. I was looking for something that I could use to quickly remove my Yubikey from my lanyard for work.

2 magnets are used on each side. No glue is needed.

V1 - on version 1 of the female coupler, there is small gap between the male/female couplers, which allows it to reconnect easier.

V2 - on version 2 of the female coupler, there is no gap between the couplers, which allows it to hold together better, although it may not always snap back together as easily.

V3 - on version 3 of the female and male couplers, I've added a slightly rounded edge to allow them to snap together more easily.

Slicing/Printing Instructions

• Use a brim, and directly attach it to your design
• Create a pause point directly above the magnet chamber. (12.95 mm for the female and 14.9 mm for the male coupler). Instructions for doing this on the Prusa slicer can be found here: https://help.prusa3d.com/article/insert-pause-or-custom-g-code-at-layer_120490. Make sure you right click and insert a pause and not a filament swap.
• When it pauses, insert 2 magnets into the slot and push them all the way to the bottom, being very careful not to break your print free from the bed.
• When inserting magnets in the second coupler, make sure you insert them the correct way, so they will be attracted to the other side and not repelled!!

I've included both STL's, and my Fusion360 files, in case you want to tweak any of the sizes.

Enjoy!!