This credit card holder is open on one side and has a window in the middle of both sides to make it possible to flip out the card needed. The clever part is the bending of the window to the inside, so that the cards are being held tight and cannot just slip out. It can hold
I created a box in 3d software and solidified all faces. Then I designed the organic holes pattern and cut it into front and back. The exported STL showed some troubled walls in the heat map. I made these parts a little stronger until a flawless print was possible.
I opted for yellow nylon plastic for its hard but flexible structure and had it 3d printed. You can see the result in the picture.
Do you want one? Find it in Accessories.