you will want the 16oz, the smaller one is really only good for sprint races. also freeze the ice into big blocks in those plastic food storage containers. keeps the ice from melting as easily.

Honestly its probably cheaper to get in better shape. I know before I lost weight I was tired after a 45 minute stint in the car. Since I have gotten in shape I did an hour and a half stint at Daytona in ~85* weather and felt fine afterwards. No cool shirt. I have the system from my dad's old race car, but haven't bothered to put it in mine yet because I haven't had the need.