Our cat is forever losing his (very much over-priced) collar magnets that trip the latch on his cat-flap.

This thing works as a cheap replacement when a 'standard' fridge magnet (17.5mm diameter + 5mm thick) is inserted + hot-glued.

Note that the magnet slot is deliberately several millimeters taller than the magnet in order to allow for the inevitable intrusion of printed support material.

Thing dimensions: base = 23mm square; height = 22mm.