I have one of these in 12x18. It does a great job helping keep the inside of the car cool. I like that it allows some airflow through it.
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Just about everyone I train with use these too: RYOBI 18-Volt ONE+ Hybrid Portable Fan (Tool Only)-P3320 - The Home Depot
The only bad thing about the fan is you have to buy batteries separate, and they are pretty expensive. I carry three with me so I can ego for hours and hours and not worry about it dying. You can also run it off a regular 120v outlet with an extension cord.