Here is a fairly simple card wallet design which utilises a leaf spring to secure the cards in place. It can store 6 or 7 normal bank cards, or a similar number of business cards that are 85x54mm max.

0.4mm nozzle
0.2mm layer height

The leaf spring should ideally be printed using PETG although PLA works fine, and I strongly advise using a brim to secure it in place during printing.
The two case halves can be printed in any material and will need a little bit of superglue to hold them together, I recommend the gel type.

Supports aren't required.