A tiny 1/4" bit holder with a keyring cutout.

The hex handle follows the shape of the bit for grip and minimal size.

Uses friction and an optional Ø6x3 magnet to keep the bit in place. Some bits fit looser and some tighter, which might need pliers to remove.
The FreeCAD file has a spreadsheet to change the dimensions, or ask and I'll upload new ones.

To add the magnet, I suggest sticking it on the bottom of a bit, and pressing it in through a screw. Check the photo to see if the bit is seated correctly.

The example is printed in ASA with 0.4 nozzle at 0.2 layer height with fuzzy skin (0.1 thick, 0.25 distance) for extra grip. The base has enough adhesion to print standing up on a smooth PEI sheet.