I designed this case to fit a Raspberry Pi 3. I left a lot of room for airflow, but also some protection. All ports should be accessible in this model.
It's held together with 4 M2x8 screws. The inner support posts have holes to accept the screws and are slightly undersized, so the first time the screws are turned in, it creates threads. I printed this in PLA plastic, so the threads are not super strong, but plenty to keep the top on.
The case in the images was printed with .15mm layers, and a little bit of support material in the openings, however depending on your printer, you might get away with bridging these gaps.