Going with 2x engine cooling fans from a local carparts warehouse.
14" 2200rpm 80watt 12V - 530cfm, so 1060cfm total.
70$ new for both so fairly cheap, hopefully that is enough and if not a third will be added for 1590cfm total.
I tried these fans in the GPU days in 2012. They are extremely fucking loud and they get very hot. I had to disconnect them after a few days because they smelled like burning. I don't think they are meant to be run all the time. I used gable fans from home dept instead. 2200CFM.
They are simply not meant for this usage.
Btw, like these setups, but I still like the ghetto 100 GPU 5870 setups better hahaha...