Some 4mm bit holders compatible with Riffnshred's excellent Pocket size driver bit case.
Works best in PETG for the flexibility, but PLA works okay if you don't mind the last few bits being a bit hard to push in.
Print the 2 different bit holders and the spacer, then assemble as per Riffnshred's original instructions, using the spacer to replace the ninja star. One of the bit holders is rotated by 15 degrees which should give just enough space for standard 28mm bits.
If you bits are longer or the working end vs hex ratio favours the hex end, you may have better results installing the holders upside down so it's easier to get the bit you need out. Experiment!
The author remixed this model.