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 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.
Word of advice, next time before you decide doing something, take some time and research a bit more about the thing you are interested in. That way you will prevent most of mistakes.
But I don't think you made big mistake with this choice. It will require some money for more fans yes, but at the end all of that invested effort will pay out. Also, I guess you wanted closed rig for a reason and not only because it looks badass
