I've been working on this design on and off for a few months, and I'm finally happy with it. It can store seven DF500 bits along with the wrench in a convenient form factor.
https://www.printables.com/model/509954-festool-domino-micro-systainer-bit-case/files
https://www.printables.com/model/509954-festool-domino-micro-systainer-bit-case/files