I adapted my other bit driver design to be a lot smaller and added a hole for a keyring. It's probably not going to win a torque competition, but it's solid enough to get you out of a pinch. Actually I was surprised how much torque this thing can generate in its top orientation.
Print with 100% infill and 2 walls. I prefer .15mm or lower layer height for accuracy and layer adhesion, but .2 has worked fine too. A brim is generally a good idea as the corners will want to life a bit, but not really mandatory. Arachene walls is a good idea too.
The hole for the magnets is slightly oversized to fit a 6mm diameter magnet. The hole is also 2mm deep, so a 6x2mm magnet should be perfect. A 6x3mm will work too, but will mean the bits won't fit in the hole quite as far. Actually pretty much any magnet no larger than 6mm diameter is probably fine.
Enjoy!