http://www.homedepot.com/p/Lasko-Weather-Shield-Select-20-in-3-Speed-Box-Fan-with-Thermostat-B20570/205596323
I will post how these help. The thermostat looks like a good idea.
I had a few of those Phil.. They worked great for a bout a month.. the grease oozed out of the motor and locked up. But i also had a tremendous amount of heat coming from the back of all those machines. I also had one at my first farm using it as an intake since it was "waterproof" It died aswell after about three months of continuous use. Make sure you keep an eye on them during your hottest days. We just had a 96f day with 83% humidity and my rigs kept on humming. gotta keep em clean tho! http://www.ruralking.com/ventamatic-maxxair-24-yellow-direct-drive-tilt-fan-bf24tf.html im using 2 of these stacked one on top of another ported thru the wall.
I have tried a few fan's out the maxair one I had got worked fine and seemed like decent quality but I was not getting the 4000 cfm they mention highend. I have one http://www.lowes.com/pd/Utilitech-Pro-24-in-2-Speed-High-Velocity-Fan/4755305 had two but one gave out. Lowes had some kinda policy you could call and get a new moter for X amount of day's from purchase but they were out so I ended up getting refund and bought one bigger as they were out of 24 inch fans at the time http://www.lowes.com/pd/Utilitech-Pro-36-in-2-Speed-High-Velocity-Fan/4755307 . It really can move some air though.
Between those two I am able to make it through a midwest summer. My mining area I have the hot air going out a gable fan the one that was installed there did not cut it http://www.homedepot.com/p/QuietCool-Professional-3013-CFM-Power-Gable-Mount-Attic-Fan-AFG-PRO-3-0/206047381 was able to move a lot more air out. But that setup was close to what made it through last summer for me and is doing good again this summer.