I just drill a small hole though the top, run a zip tie through making a loop, and then clip to the front of the crate. There is a high and low setting, they don't eat batteries, and you can run them off your car battery with an adapater. I think the air flow is much better than any of the crate fans on the market I have tried. A friend of mine recently found them at Walmart, so you might want to check there.
The o2Cool fans will deplete your car battery if left on long enough. Seen it happen. Their batteries don't last long enough for my liking. I have a brand new one I never used because I switched brands.
The Ryobi fans now have an optional battery that will run the fan on high all day. I opt for Ryobi. I have four fans and 8 batteries. They also have a 6 battery charger. They have a stand and hooks so you can position them several ways.
These are great. They are big (10") and sturdy and have a good strong breeze. They run on 8 D batteries but that lasts 40 hours. They can also run on an electric adaptor. The ones I bought a year ago have a hook on top that makes it easy to attach to a crate. I can't tell by the picture whether these have a hook but they would still be easy to attach. They will also stand freely so you can use them for more than one crate if it's not too hot.