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.
There will be a video of the design/printing of this case over on my YouTube channel http://www.youtube.com/diodepress along with my other 3D printing projects.
UPDATE June 30, 2017:
I've added a new SNAP fit version of this case to the listing, as well as an option to mounting onto the LulzBot Mini printer. This eliminates the need for the M2 screws.
Top of snap-fit case: diodepress-raspberrypi-SNAP-case-top.stl
Bottom of snap-fit case: diodepress-raspberrypi-SNAP-case-bottom.stl
Bottom of snap-fit case WITH frame mounting for lulzbot mini: diodepress-raspberrypi-SNAP-case-Bottom_LulzbotMount.stl
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.