basic multi-card pocket wallet
Started with the design of thing:1661552, and took the middle part and turned it into a back, then opened up the space in the middle. This should hold 4 or possibly 5 credit card sized cards. For many of us, that would be along the lines of an ID or Membership card, a Drivers License, a credit card, and possibly a bus pass. However you could fill the back with a sheaf of paper and use it as a tiny pocket notebook. Again 6mm thick, rounded edges, carry in pocket.
This thing was made with Tinkercad. Edit it online https://www.tinkercad.com/things/iq1xSviM2MR