I found this on Amazon. One of the reviews even mentions using it for Pokemon Go. Relatively cheap, too. I know it's not a shield, but it might help.

https://www.amazon.com/VIVISKY-Portable-Cooling-Holder-Suitable/dp/B071DY7QN5/ref=sr_1_4?ie=UTF8&qid=1526487858&sr=8-4&keywords=phone+cooler