This is a remix of a model by ToysByThomas. I cut the keychain loop off and added two holes for a 3x1 mm magnet.

I added a pause in the print profile so the printer will pause when you need to add the magnet. I placed a drop of superglue in the hole before adding them.

This model requires two 6x3mm magnets.

Small Magnets, 120 Pack Refrigerator Magnets 6x3mm Rare Earth Magnets Neodymium Magnets for Crafts, Whiteboard, Kitchen Cabinet: Amazon.com: Industrial & Scientific