For quite sime time I used https://www.printables.com/model/73715 as my regular wallet. With the inspirations of that one and some additional inspirations from https://www.printables.com/model/242551 I designed this model that only requires a single print and one single rubber band to hold a large number of cards. I used a silicon hair band but any similar sized rubber band should do.

I printed the model in Prusament PETG with 20% infill and Prusa default settings for 0.2mm layer height otherwise, but the precise material and settings likely shouldn't really matter with that design. No supports are needed.

The STL file attached is for a silicon hair band with a diameter of 3mm. For other sizes you may modify the parameters of the linked OpenSCAD files to change the dimensions of the carve-outs for the rubber band.

You may combine this with credit-card-sized coin holders like the one I designed for Euro coins at https://www.printables.com/model/73752 or the variant for UAE Dirham coins at https://www.printables.com/model/157074.

Update: There is a new version that instead of being solid has two compartments for very small extras. I use them to store extra SIM cards or MicroSD cards. Those compartments are closing very tight, which is intentional. If you don't need the extra compartments you may want to use the original solid version.