I first saw the idea to use a fan on the Vans forum, Jim Stricker posted he was doing it on his -12.
The concept is to use the fan to pull the hot air out of the cowl after the engine is shut down. The benefit on hot days is cooler under cowl temps for the first part of the flight and lowering the fuel line temps helping to avoid vapor lock.
My approach was to use a 120mm 88 cfm computer fan and power it with a 12 volt, 2200mah LiPo battery, because I wanted the fan to be completely isolated from the airplane power. Another power option would have been to plug the fan into the battery charge pigtail accessible inside the oil door. The battery is a standard RC model battery and will run the fan for 6+ hours on a charge. The fan is very effective, on a 100f day after flying it lowers the cowl temps from well over 250f to 120f in less than an hour.