OpenSCAD model for magnetic button/indicator - basically a disc with a socket for common cylindrical magnets. The STL and OpenSCAD source is for a 30mm diameter button with a 12.75x3.2mm magnet, but it's customizer-enabled so you should be able to modify it easily.

My initial use case is as a dishwasher clean/dirty indicator - I wasn't happy with any of the more complex ones I found. For that, I printed the cap in a different color so just flipping the button over indicates the state. Another idea would be to put a design on the cap and/or button body, provided they are printed with enough height (taller button or thinner magnet). A small amount of glue will hold the magnet and cap in place permanently, but I found the cap to be snug enough to hold everything when sticking it against a dishwasher or fridge. Sticking it to another magnet, however, was enough to remove the cap and magnet.