My wife wanted a magnetic catch for a bag she had made, which originally had a loop + button. I happened to have a box with a ton of 2x5 round neodymium magnets, so I came up with this. 

Simply print 2 with a “color change” or print pause before the top cover layers start and stuff 4 - 2x5 round magnets into it. The tolerance is a bit tight, the friction fit of popping the magnets in was enough to keep them from lifting out to my steel nozzle, scale as needed for your printer. Can be sewn, crocheted, etc into projects you want a lightweight magnetic catch/clasp.