Here are some nice cases shown, so I do not want to hesitate presenting mine.
Since I do not trust cardboard cases keeping my RPi save and I do not have access to to a 3D printer or laser cutter, I utilised some 3mm thick plastic board and glued to pieces together. I believe it is made of polyurethane, but I am not absolutely sure about. It is a little brittle but somehow foamy. You can cut that stuff with a - very - sharp knife and file every piece in shape. It is easier than handling wood. It took a couple of hours, most of the time waiting for the glue to dry. The case weighs less than 40 gramme.
Two little thought went into the design: the case is some millimeter higher than it needs to be, so there is some space for a RTclock, a future LCDisplay, LEDs or switches.
And I wanted to have a opening lid for eays access to the GPIOs. For now the case is hold together using rubber bands, in the future I will build in a lockbar. Probably.
I am open for any thoughts or critique.