This is a case designed for the Raspberry Pi B+ and Raspberry Pi 2 computer. The design also works for the Raspberry Pi 3, and I believe 3B+. The case is entirely passively cooled. You can print 2 base boards and four outside locks to create a case for a single Raspberry Pi, or use a combination of the baseboards and locks to connect a number of units together to form a Beowulf cluster of your chosen size. For large clusters (> 10 nodes) you may find that a fan may be necessary to assist in cooling.
This case design was originally designed for my Parallel computing course. I never ended up using the files for the course, since the Parallella board became available to purchase last July (see my other contribution for Parallella case files). We did use this design to create a 21 node raspberry pi cluster (pictured), which two of my students have used for independent study work. Neither student reported any issues with the cluster overheating. We used the case files to build a 28-node cluster of Raspberry Pi 2s, which required a fan to help with cooling.
An 8-node Raspberry Pi 3 cluster built in 2017 is fully passively cooled. Keep in mind that larger clusters (especially as the Raspberry Pi's power draw increases) may require a fan to assist with cooling. It is also recommended that for larger clusters, you add an additional layer of base boards to help increase the space between units and improve airflow. Raspberry Pi tutorials etc. can be found at this link: http://suzannejmatthews.github.io/2017/06/14/pi-cluster/
The case was co-designed by Suzanne J. Matthews and William ("Frank") Blackmon.