I've been making tons of these for family who have to be out in the heat. They have water-absorbing crystals in them and are easy to make. I've heard people say they help keep them cool for a few hours. They could be used alone or with your other cooling vests, etc. My son-in-law loves them!
Here's a link if you can sew:
If you don't sew, I'm happy to make some for you, but be patient; I'm going on a real vacation soon!