And that answer is HOT AIR AISLE
Preferably, you need to have blower style GPU as they are super easy to route hot air where you want it. It is possible to do with standard aftermarket GPU coolers but somehow you have do direct it into hot air duct, but now you need much more airflow to do so (i have no experience, all my cards are blower style)
Step 1. Put some cooling fans into your mamma basement where you normally keep your rigs.
https://i.redd.it/2vw6ks8f6hd11.jpgThis is how I did it. I got very nice EBM PAPST fans, 80w each. Superb airflow, just enough noise not to be annoying outside the house. I got them like 20e each, i put two as exhaust from the room) Made a simple wooden window, put some circuit breakers, and that is it.
STEP 2
https://i.redd.it/gkggpz0n6hd11.jpgPut your blower rigs in place and fabricate the air ducts. Make your rigs GPU and PSU and everything else blow the hot air into the duct.
On this photo, there is 41 Vegas using like 7.3KW of power, in the summer of Croatia (35C ambient and that is HOT) cooled only by 2x80w fans, and the entire assembly is using less than square meter!
https://i.redd.it/2gs8jw966hd11.jpgThis photo shows how I used cardboard to make ducts for PSU and GPU and everything. Seal everything, to ensure the duct is negative pressure and no hot air enters the room.
The result is that your rigs run super cool even in the hottest of summer, the room is cool.
Basically, the only rule is that you throw the hot air direct out of the room, if you mix it, no amount of cooling or fresh air will bring it down.
Please don´t tell me about fire hazard
I will replace all the materials with fire retardant. This is just testing level. Sorry that I don´t have many photos of progress