I think the cheapest method is just to buy a server chassis and mod it to include more industrial fans. Should be cheaper than 2x Full towers.
Rosewill R4000 or L4000 should work.
I was trying to get more case pcie "expansion" slots, but I really like those cases. too bad they don't make one with miners in mind.
I didn't want to stack the GPUs right next to each other...but maybe if I was running only 4 (1080s)
What about this:
Thermaltake Core P3
https://www.newegg.com/Product/Product.aspx?Item=N82E16811133313&cm_re=Thermaltake_Core-_-11-133-313-_-Product