This model is based on the model of mishkin2 but totally redesigned from scratch. After printing the design from mishkin2, I thought there could be some improvements made, both in design as well as in printability. 
I've used this redesign for over 3 months now with no troubles so that's what I call a succes.

Improvements made:

1. The cutout at the bottom is a bit bigger for easier pushing out your cards
2. A cutout is added at the top so you can push your cards into the wallet more easily
3. Made the front and back flat for easier printing
4. Added a small tab at the top for an easier grip (one-hand opening)
5. Split the wallet into two parts; printing is now really easy (no supports, no bridging whatsoever) and, if desired, the model can be two-color really easy now

It's not the most “fancy” desgin but front and back show you your perfect first-layer all the time, that's worth something isn't it?

I would suggest printing the wallet (or at least the back part that flexes) in PETG (more temperature resistent and more flexible than PLA).

A few drops of glue is all that's needed for assembly.

Enjoy!