Hi everyone,
I'm a CS professor that taught a parallel computing course with the Parallella last semester. As part of the prep for my course, I co-designed some case files that all my students used. You can access the files from Thingiverse at this link:
http://www.thingiverse.com/thing:892684
The students used the case all of last semester and reported no issues with cooling. One of the nice things about the case are the connectors -- using them, you can create a cluster of parallellas that can be built horizontally or vertically. I demo'd this cluster and case design at SIGCSE'15 back in March.
Enjoy!
-Suzanne