open air for me too.
GPUs are zip tied to racks.
the room is the case and the windows are the exhaust.
Started my GPU farms like the above in 2016 and never looked back - ease of maintenance is the key.
Dust is the only pitfall though - I gave up on this long time ago and most of my 470/480 rigs still running at peak non-stop eventhough they are bloody dusty and grimy.
Well ..... as long as critical parts still works fine like fans for PSU, GPU and CPU - will just leave them alone. All of them are well ROIed several times over, so just leave them alone.