I use a cooling vest I got from cycle gear. Soak is and it will give about 2 hours of evaporation cooling. It works great as long as you are moving. I keep it rolled up in a dry bag. I add water to the dry bag and let it soak. I wring out excess water as I remove it from the dry bag. There is usually enough water left in the dry bag to re-wet the vest a second time. I have in the past, stopped by a mountain stream and re-soaked it. I had to put a rock on it to keep it submerged. It take 15 to 20 minutes to get the vest to absorb enough water for a couple hours of cooling. I put it on over a synthetic t-shirt. Initially it feels clammy, but once moving it makes riding on 100F day tolerable.