I designed the MAG wallet (short for CC magazine) because I was tired of looking for an efficient small wallet that would hold all my cards plus cash, plus other EDC items and was easy to get to whatever credit card you need.
It has slots on the sides for adding slide on modules like a laynard attachment or a module that covers the back (or front) etc.
You can download the .obj print file from here: MAG Wallet Download
Note that regular PLA works great and it needs to be printed with supports.
You'll need to print with supports enabled. The green parts are the supports that your slicing software will generate that you need to pull off after the print is complete.