This bit holder is designed for use with 30mmx1.8mm round magnets. It is designed to have a magnet behind and in front of the bits and it can also be used with a third magnet behind your shirt to hold it to your shirt. The magnet in front should be glued in place and can double as a screw holder. This was printed on my Ender 3 standing upright so as to avoid supports in the cavities.
Download from thingiverse