Thought I would share the custom 4U rackmount-able case my brother and I customized to support a 6 GPU rig.
It does an excellent job of cooling due to it having 3x 120mm intake fans, 3x 120mm fans pulling air out of the GPU compartment and 2x 80mm fans pulling air out from the motherboard compartment. We've found that setting the rig up this way makes it much easier to keep the GPUs running cooler compared to my open air rig since the ambient temperature around the open air rig is much higher unless a desk fan is placed to direct the hot air away.
Customizing the 4U case was tricky for numerous reasons
1) The internal set of fans was mounted too far forward and had to be moved back to make room for the GPUs
2) The intake fans were mounted to the hard drive bay, which had to be removed. We installed custom rails to replace the removed hard drive bay.
3) Creation/tapping of the GPU mounting bars and support beam.
We've built a couple of rigs in cases like these so far and can say we'm quite happy with the results. We're going to see how a similar setup with 8 GPUs works soon.
Let me know what you guys think!
The final modified case, GPU screws tapped (threaded) into aluminum beamFront View, no doorFront View, with door and lidOne 1070 installed for referenceFully assembled, lid offFully assembled, lid on, door up