I have bought several cheap-ass Packard Bell keyboards with MX-compatible caps. They are pretty poor, but good for experiments.
There is a little plunger type of stem that has 4 "feet" and is loose when you break the board open. You could harvest 100+ of them per board, along with the keycaps, and hot-glue the stems to magnets. The entire assembly would end up being fairly tall, but it would be easy.
I also epoxied them to a baking pan to make a Retro-briting tray.