Took off the bezels, compressed 4 into a block, had a rigid bifurcation card link GPUs 1 and 3 on top, another underneath the block do 2 and 4.

Then I fitted my own custom 40x40x30 fan shroud for 2x6000 rpm fans at one end.

The other 2 I did something similar for but a single 2 card block and a stock fan shroud online.

Oh and also side mounted a 4GB 750Ti for graphics, and/or whisper use, parallel to the mainboard. Airflow from the front fans to back is unimpeded by anything in this way.