My current 1080ti FE/Ref types 2-3 GPU rigs stays cool with a blower to suck the air out faster. The non-FE cards, maybe a bit tricky because the only effective way for my case is to hang them with hooks (heat goes up) with ample spacing.

Be careful when you select which fans to use because anything below 3000rpm "typhoon class" type fans are not going to make the cut - the Scythe fan that I am using now is a 3000rpm server rated fan (from the Antminer S7 days). Very good and powerful fans. There are better ones like Noctua, Delta, Nidec, etc. but more expensive.
https://www.amazon.com/Scythe-DFS123812H-3000-Ultra-Kaze-120mm/dp/B001JKNMBE/ref=lp_2530362011_1_3?srs=2530362011&ie=UTF8&qid=1498738110&sr=8-3

The "before" pix below.... all the 2-3 GPU rigs will be consolidated to 3/4/5 x GPU rigs.... when I am done with that project - will post "after" pix.

The Zotaz 1070 Minis are attached to this rig because waiting for the Asus Prime Z270A motherboard for 7+2 x GPU. While waiting ... they work hard along side the Zotac 1080ti FE

Disclaimer: During the PSU supply crunch, I got a few "Ethereum PSU" from China.... until I get the EVGA-G3's and server PSUs.
So far so good but I am not very comfortable with the "poor man's PSU" so I use them when only needed and did not beyond 50% of the alleged watts rating. It however got all the cables, connectors required up to 6 x GPUs (6/8pins) or 3 x GPU (2x8pins) -- its the real deal.... but I don't recommend it unless you don't have any other choice.