Jesse76 has designed a great Raspberry Pi cluster tower, missing, in my opinion, only a section for a fan. I've designed one to address that minor issue.

This is a remix - I pulled jesse76's original no-vent lid into OpenSCAD and proceeded to do horrible things to it - cutting and stretching and drilling - finally assembling the ragged pieces into a working fan section. Jesse's nice "Multi"-(pi-symbol) logo survived the process intact.

I made one each for 40mm and 50mm axial fans with an optional switch mount. I may add one for a blower fan in the future - only for completeness; the axial fans work fine (though they're a bit noisy.)

I also carved a little space (~.5mm) out of the connector area so the connectors fit a little better letting the top snap into place more easily.

You may notice I included a photo of a broken microSD card and that the fan section doesn't use one. I just wanted to let you know that it can happen to you if you're not careful. It happened to me when I was removing an installed Raspi from one of the cluster sections without first removing the SD card. Snapped the end right off - and then I snapped the rest of it with my fingers - much easier to do than I expected; they're quite fragile, apparently. Anyway, I won't forget to first remove the card any more.

[Edit] I forgot to mention: The holes for mounting the fan are sized for M3 screws. I use M3-14 button-head screws - the heads fit down into the nut-slots in the fan housing. If you use different screws, you may need a few more mm in length. Once through the fan, your screw has about 5 more mm of plastic to bite into - it doesn't take much to hold these little fans in place - maybe a turn or two. Also, watch out that the screw heads don't contact anything on the Raspi that might not appreciate it.