Based on the Hardcase wallet (and the use of it) and a pre-assembled hinge, I designed this "Folding wallet cassette". The idea is that cards go in one side and cards + money goes on the other side. Just printed version 4 and I'm getting real proud of it! Some features:

• Holds 4 cards on one side (2 in slim version)
• Holds cards, money, small change and other things on the other side
• Looks great
• Great usability with minimalistic design

The design goal is minimalism as well as usability. When I first saw a printable hardcase wallet I instantly had the urge to print it. So small, yet it can contain cards, small change and paper money. I decided to make my own parametric version in OpenSCAD and started using one.
I tend to have about 5 cards and small change + paper money. It was soon clear that my 5 cards were impractical, especially when having loose money too. After fiddling with it I decided to make a foldable version with cards on one side, while having the money on the other side. I saw the preassembled hinge and also fell in love with that.

The first version has two holes on the top. The cards slide in sideways. The body was cut in half, except for the long side - on one half it has the closing lid while the other side has an opening to slide the cards out. It looks like a cassette tape hence the name.

Problems with v1 is that you could not easily find the card you need. Also small change tend to fall out because the side is divided in half. A small opening (because stashing it too full) causes coins falling out.

Versions 2 and 3 have cosmetic changes, so let's skip to v4. Cards go in and out vertically, so I can use my left thumb to scroll through the cards. The short end "walls" are on the right side. When the wallet is not fully closed, it is harder for coins to fall out (they still can through the long end). The closing lids are now small cylinders at both sides, they work quite well. The right side has a hole to push cards out. I experimented with the left side decorations, I think I like the off-center circle most. Right now I'm very content with this version. I'm so proud what I even branded it :-)

So... enjoy and print your own wallet!

Edit: after extensive usage I've come up with v5. Some flimsy parts are slightly thicker. The inside has a tapered edge, to prevent clamping in coins.

Edit: version 5d has some more minor tweaks.

Edit: v5e_12mm is a slightly bigger version (1mm thicker), which fits one extra card and maybe a few more coins.

I printed this wallet with a 0.35mm nozzle, first layer 0.25mm, other layers 0.2mm, 0.3 infill, extrusion width 0.45mm. ABS. It takes about 3.5 hours to complete. No support or whatsoever.
Carefully break loose the hinge at the bottom side. Not much force is needed. Also sand the closing lids a little bit so it closes and opens easy enough (probably during use they will get smoother too)... done!
v1: The hinges respect the 45 degrees overhang, as the bottom was "rounded" by an 45 degrees angle. Ugly though. The roundings do not respect this rule, but it prints fine.
v4: The hinges nor the roundings respect the 45 degrees overhang rule. But they print fine on my printer. In older versions the hinge has a bigger and a smaller part. The smaller part prints fine while the bigger one was not perfect. Now they are almost (visibly) identical and it prints better.
I printed a slim version with 8 hinges and it gave a non-manifold error (on the hinge, I don't know why), but it prints fine. I uploaded a 9 hinge-version which does not have a non-manifold error. Should print fine.
I haven't printed the fat version yet. The hinges may have to be adjusted.
Version v4c has the following improvements:

• Hinge has about equal sized sides on both sides. This let's my printer print the overhang better. It looks better too.
• The head sides are now living on the "right side" only. Previously the sides were cut in half and it resulted in coins falling out.
• The cards are now placed in vertically. You can use your left thumb to scroll through the avaiable cards. It holds 4 cards.
• The closing lids work quite well. You may need to sand it just a tiny bit.
• Better roundings and decoration.
  You need my library to use the scad file:
  https://github.com/jackha/3d-models/tree/master/library