It's a stir bar made to fit a 3/16" x .75" cylindrical magnet. I have a stack of four 3/16" x 3/16" magnets inside one. Great companion for my stir plate!
Let me know if you want different size capsules, or to use different size magnets.
Not yet tested.
The commercially-available ones are made from PTFE and come quite cheap. I would recommend purchasing those. If you're just stuck without one and need one quick and have the magnets on hand and a 3D printer, have a go!
Should be built with nylon, but I'm not sure how you glue it together; perhaps heat-welding will work well. Print with supports.
If made with ABS and solvent-welded with MEK or Acetone, be damn sure they are completely dry/cured/devoid of any ketones, before you use this on anything that will go anywhere near your mouth. On second thought, just don't use this with ABS. Seriously, what's wrong with you?