Oh that would be awesome (and a guaranteed purchase on my side)
If you do make a top for the modular case that has a opening and some holes to mount a 40mm fan (like the Noctua NF-A4 x 10-FLX 5) that would be grand!
Looking forward to it very much
btw: in the meantime, while i made no progress on the cooling side, i got the whole setup rockin'.
The cluster runs in a Bridge/NAT hybrid mode - a bridge configured on the Pi3 connecting all of the Pi-Zero's together and then a NAT between the bridge (br0) and the network port of the Pi3 (eth0). It runs automated headless tests of various things. The Pi3 fetches tasks from a shared drive of my main computer and then gives these Tasks to the Zero's to chew on and in case they are all busy, the main Pi3 itself can also run up to 4 Tasks at a time, meaning i can run up to 8 automated tasks parallel (whereas on the original virtual desktops we used for automation we only had 2 very unstable Tasks parallel).
Heat still is a minor issues. If all Pi's are working their hardest, they do tend to start reducing CPU speed to cope with the increased temperature.
Overall I am very happy with it