Mining is always at least with open case.
Preferably on open air, PCI-e risers and an household fan blowing at the cards. You can skip one the 3 before IF you seriously undervolt cards and monitor temps, and even then it's not recommended.
The way you're doing it, cards don't last long.
Slightly off-topic here, but what are the estimates on how much mining shortens the lifespan of a card?
I have an nvidia gtx 680, if I leave the fan speeds at auto it'll go to 83-84C. If I set the fans at full, it'll drop to 68-70C. I guess I should be running the fan at full speed when mining?
I'm investigating on leading cold air to the computer. Since it's currently -15 to -20C here for many weeks to come, and many months of below 0, it would be beneficial. I have to study and experiment a bit on moisture to make sure it doesn't start stacking up inside the computer.