[UPDATE]:
This is no longer a difficult print! I have separated the top section into two pieces that fit together easily in post-processing. A couple drops of super glue will hold it in place or forget the adhesive and use as a removable cover. As some suggested, I have also added a model with an area for GPIO access!

This is a Raspberry Pi 3 B/B+ case that was built to allow a Noctua NF A4x20 5V fan to be installed internally. The fan is mounted at a 5 degree angle to help push the air out of the vents located on the side.

Aside from the fan (https://www.amazon.com/Noctua-NF-A4x20-PWM-premium-quality-quiet/dp/B071W93333/ref=sr_1_4?ie=UTF8&qid=1533694145&sr=8-4&keywords=noctua+nfa4x20), you will need:

8 M3 20mm screws
8 M3 nuts

All screws and nuts have pre-sunk locations on the model, which should make installation easy enough.

I have included plastic towers inside the model that the board fits onto and locks it in place when the case is screwed together. Remember, you are working with plastic so don't over-tighten.

To install the fan, I cut the yellow wire (carries PWM signal) and connected the red wire to the 5V pin on the board and the black wire to the Ground pin. If you are more ambitious, you could easily write a script to control the fan based on internal temperature using the yellow wire that a snipped attached to a GPIO pin.

Prusa

Doesn't Matter

0.1-0.2mm

Designed using Fusion360