This is a case I modeled loosely after the style of the electronics case of a Prusa i3 model printer. The problem I was trying to solve had to do with needing to place a fan on the Pi for use inside my printer enclosure.
You will need to pick a bottom and a top to print. The differences in the bottoms have to do with how the case will mount. The different top versions have more to do with how the case is ventilated. The FAN mount top includes a recess to accept a 30x30x8mm standard Pi fan. There is also a passive ventilated top, and a top with no ventilation as well.
The holes in the case are set up to work with m2.5 screws, just like the holes in the Pi itself. The nut recesses in the bottom of the case are intentionally set to provide a tight fit. Use a screw to pull an m2.5 nut into each recess and you can be confident they will not fall out while you install the case itself.
You will notice notches built into the FAN top for the wires from the fan. You can either have it installed label side in to blow air over the pi (and wrapping the wire over the top of the fan, through the channel and into the case) or label side out, drawing air from inside the case and out through the fan. In the latter case, the wire can enter the case directly through the internal notch in the bottom of the fan tray.
The longer slot in the top of all the versions of the top of the case are intended to provide a place for the camera ribbon cable to exit the case.
Feel free to suggest other changes or other configurations for mounting the case or for additional accessories. As I run across a need for different setups, I'll add more parts too.
Supports are only really necessary in the fan cavity. I like to run them to keep the nut and screw recesses clean.