We have gone to WDW in Aug for the past couple of years. I do not do well with severe heat. So, I took 2 Chill Towels, sewed them together, and made pockets in them. I wet the towels and then pack the little pockets with ice. I then added velcro at various points so that I can wear is around shoulders or waist, or whetever. I made one for each of us. We look completely ridiculous, and everyone stares at us, but I don't care at all. I just keep refilling the ice, and I am cool all day, even in August in FL.
Alternatively, you can buy a real "cooling vest: at a medical supply store, but the ones that actually work can run $300 + , and they can be heavy.
Have fun!