Another way is to try to lower down room temperature! It actually helps a lot for closed rigs
I guess that in some cases that would be good thing to do. But in OP's case, lowering room's temperature is not necessary. At some point he can do that, but it would require more money. On the other hand, he can install few exhaust fans and his problem will be solved for less money. Not to mention that electricity consumption of AC and exhaust fans is not the same.
Of course if I had the money and place for AC then there would be no need for making it a closed rig. People are saying to lower the room temp like it's very easy. I would have already done it if there was a way. I am gonna add another fan in front and see what happens.
One thing for sure I will never make another closed rig. Money spent on fans and sheet and my labour + more electricity bill + more noise = not worth it.
But my rig looks badass.