I know summer is close to wrapping up but just thought of adding something.
The vest coolers are very nice and indeed add much needed comfort, however from a heat safety standpoint they are not the most efficient cooling systems you could be using. We all know that best ways to cool down in a hurry is to cool your neck, this is due to the fact that we have major blood vessels (carotid artery) which are very close to the surface of our skin in this area of our body. If you cool down your neck you are getting the biggest "bang for your buck" with all the blood that you're able to cool in a relatively short time.
I say all this because you can find a simple neck wrap for $10-$15 just about anywhere. Dunk them in water and wrap your neck. They come in black and worn with a shirt and mask would be hard to see, or you could at least wear them between innings. I grew up in a part of the world where 120f was normal and now live in north Texas (and work outside for a living wearing 30 pounds of gear everyday), it's hot, umpires are a highly susceptible group. Please take precautions. And of course drink water and avoid caffeine and alcohol, both of which dehydrate you.
Sorry for the rant, just want everyone to be safe out there so we don't have anymore falling out due to heat related issues.