I had 2 at one time. I had fans, and windows, but it was still too hot. The thing that really got the temps down was the misting system. The plants loved it too. It would be easy for a small greenhouse like yours, mine was 12' x24', and my big one was 24' x 48'. I used the misting system in the big one, it was like a rainforest. I loved going in and messing with my plants, a little tropical paradise in my own back yard.
Just noticed you are in Enid. I am in Bixby. Can I come play in your greenhouse? I really miss mine. I have all sorts of tropical plants that I have gathered from lots of different places. They are living (I hope) under a tree at my Mom's house right now. Not sure what I am going to do when it starts getting cold.