Super simple and very cool. A magnet encased in a printed part that goes on a keying. I first wanted to pause the print and let the printed encase the magnet in one print but the magnet kept getting pulled out by the extruder carriage. So I printed 2 tiny parts in ABS and acetone welded them together then smoothed them. Its very simple and its very helpful especially I I cant put a key holder on the wall in my rented flat. Now I just lob my keys at the metal lights witch and they (almost always) stick. I used a 6x5 mm Nd Magnet. The CAD can easily be modified to accommodate a different sized magnet.

https://www.thingiverse.com/thing:2781766