Nes controller hat pin or refrigerator magnet.

thingiverse

Uses 6, 8, or 10mm magnets. May also be used with bar pins. For a hat pin you probably want to use neodymium magnets for maximum hold. For the fridge magnet you should be good with any magnet that is the right size. The one without a magnet hole would work great with bar pins. The magnets work really well though especially the double hole one. This was my first design so if you blow it up there maybe imperfections. The one in the picture that is painted is an earlier version. The unpainted one is the one I uploaded, I just haven't got around to painting it. I designed this so it would be easy to paint and have crisp lines. all the painted parts sink in. When you are done painting you just sand the top flat to remove any paint on the grey bits. You probably don't want to go over .12mm layer height. If you get gaps on the top layer your paint job wont look that great because the paint will go in the gaps. Try to minimize gaps, on the top layer especially. A couple of the backs are upside down. Flip the backs magnet-hole side up in your silcer. TLDR: Uses strong 6, 8, or 10mm magnets. May also be used with bar pins. Paint then sand top layer on flat surface to remove paint from grey areas. Use .12mm layer height or lower. Minimize gaps on top layer for best paint job. If needed flip the backs magnet-hole side up in your slicer for best print.