I’ve got a small mining setup and the one thing I can tell you from experience is that you need air flow through the room. If you don’t, regardless of what fans you have in front of the GPUs, a “sealed” room or one without enough airflow is going to get hot very quickly.
Even opening a door doesn’t necessarily get it cooler - in my opinion you need something like a window at one end, and a window/door at the other. Air will pass through then. For me, this has allowed the air temperature (not GPU temperature) to drop from 45C to 35C within a few minutes.
Using box fans will work for small setups, but if you think of them of trying to hurry along/speed up cooler air passing over the GPUs to cool them - but once the GPUs heat up the air, you need ther air to find a way out of the room or it will recirculate and get temperature will continue to increase.