I've been looking for a bit holder for my Milwaukee M12 drill/driver but all the ones I could find were for the Fuel versions which I don't have, so I designed my own. 

It requires a M2.5x8 screw to attach to the drill and a 20x5x2mm magnet, but I've included a parameterised Fusion 360 project so you can change to a different size magnet if required.

I printed with Prusament PLA on a Mk4 and it bridged no problems so did not need supports.

Version 2

Added a couple of enhancements after using for a couple of weeks:

1. Beefed up the screw tab as mine broke
2. Added small blocks that go down into the ventilation slots that stop it moving up and down with use. These blocks work with my M12 BDDX but I cannot test on any other models as I don't have access to them