I don't get the 100+ temps often, but occasionally hit the mid 90's. I freeze two 1.0L bottles of water and drink from them as the ice melts. I also have two neck cooling wraps that I wrap around the frozen waters and alternate. I just recently bought a long sleeve "Fishing Shirt" that has mesh under the arms and along some of the back area. I've worn it once and it worked great. Will not hold up well as protection from road rash, but it's a trade off.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1