The base has holes to store spare bits in. It prints without support as the top of the holder holes are conical.

This style gives more torque/grip than the rounded style.
The solid holes and friction fit make it quite unforgiving on size, I scaled to 1.13 in the end (printing in PLA) and it is just about right. A few push pulls in and out holding the bit with pliers and it is about right.
For my next one I will probably put a springy side on the internal hex and oversize it slightly.

Thanks for the inspiration AlCampCW, your source is in there somewhere :) I wanted as small as possible but still with leverage and ability to hold a few bits. Being flat it is easy to tuck away.