I wear a neck cooler from a company called 661. It's available from most of the catalogs that sell to off road or dirtbike riders. It's called a 661 Core Cooler. I ordered a large, but it is too large. I ordered an adult small, but have not worn it yet.
You soak it in water for 1/2 hour or so and wear it. As it evaporates, it cools ya. When it get really hot, I found that by lunch time it was starting to get warm. The place where I eat lunch let me put it in their freezer while I was eating lunch and it really cooled me off well. I keep it in the fridge over night, so it is cold when I put it on in the morning.
I showed the Core Cooler to some guys that were roofing and the following week, they all had one.
I also carry a large igloo lunch cooler. During the summer, I carry 4 qt water bottles and on really hot days, I had a ziploc bag of ice and a towel. I use the ice towel to cool off.
If I go by a kid's swimming pool, even better. I dunk my whole head in, towel off the major drips and away I go. Adds about a 1/2 hour of comfort to the day.
TB