This is my take on a compact credit card case for lightweight everyday carry.
A traditional wallet is thick because of the layers of fabric and leather between the cards. I designed this case after trying other printable wallets that left the edges of the cards exposed. That made my paper cards curl and fray at the edges. I wanted them to be better protected, and for the wallet to have a cleaner appearance.
This wallet is made of two identical sides that are symmetrical, so that they nestle together. The corners and sides act like a hinge and keep the cards contained when you open one side to select a card from within. The design expands from about six to twelve cards. I like to throw a credit-card pocket tool inside when I don't want to carry a separate pocket knife. A simple rubber band or hair band holds the wallet closed, and serves as a money clip.
Update 2016 Sep 30: STL file has been updated to fix structural errors.
Best strength for the teeth is achieved by using small layer heights for a greater extrusion width-to-height ratio. I used .15mm layers.
Make sure you have good bed adhesion to keep the print from curling. Set your slicer to add a brim if you have a problem.
You may scale the design vertically to accommodate more or fewer cards if you need.