Do yourselves a favor, and take a hint from the soldiers in the desert, they need to be protected from flying bullets, but, to help themselves cool off, they'll soak towels, or, better, if they can get them, those cooling bandannas, place them around their necks, and near their temples. Those are the places where the blood vessels are closest to the surface of the skin, cooling the core is really the best to prevent heat injuries, heat exhaustion, and potentially lethal, heat stroke. I had heat stroke when I was in basic training, the treatment, at that time, was to get the temperature, in my case, 105 F down as quickly as possible, in my case they plunged me into a large bathtub shaped tank full of ice and water. It, obviously, worked, but I felt like I was a jumbo beer bottle, but I felt better after I got out. Those who've had heat stroke are more prone to it for the rest of their lives, from what I've been told, I try not to let myself get that overheated anymore. I saw someone mention evaporative garments, I've got a couple, and tend to use them when weather requires it, which usually starts in late May, early June here in South Carolina, I keep one in my trunk, along with a couple of bottles of water to soak it with. Putting the cooling vest under a mesh jacket seems very counter-productive to me, the way they work is by evaporating the moisture in them, if we block the full movement of air across the surface of the vest, or jacket, or belt, then all you've got is a wet garment under a minimal evaporative surface. Your core temperature is what your goal is to reduce, or at least keep down in the 2 digit range.
Doc