In my opinion these rigs are all very badly designed because only the first card will be kept cool while the ones behind it will overheat.
The best is to use a large milk crate or shopping cart where you can hang the GPUs at different heights. So one GPU isn't feeding hot air into the next one.
Its not been a problem with my rigs. And if you run them in a hot environment, what you typically do is have a box fan blowing between the GPU's. But with outdoor ranging at 0-10C so far its not a problem.
I need box fans in the window to create airflow however and i think i need new, less noisy one. (As in they vibrate a lot even though i'm using absorbant padding and i can hear them through the walls/ground/bed.