A box in some kind of heavy/armor/sci-fi design, with two designs of locking nuts. It's a print-in-place design (except for the nut) without supports.
It is sized to hold 2 decks of typical playing cards in their cardboard case (like Bicycle, Theory11).
I put the nuts on another build plate (in the print profile) to be easily printed in different color.
EDIT: Added a version for a single deck of cards (as requested). (Shown in the photo is the 2-decks version)