I suffer badly if my core temperature goes up... and I spend six weeks a year in India. So I spent a lot of time investigating the right sort of coolvest. I must admit I was picky, and I didn't want to look a right idiot so that influenced my choice. I also wanted something lowtech as I would have to use it in sometimes very simple Indian facilities - I could always find water, but not necessarily electricity - so the 'water dunk' models were preferred over the 'freezer pack' models.
I went with the Arctic Heat version. Not the cheapest, but affordable in the overall scheme of things. It is Australian designed but they have a US site. Their cricket team and olympic athletes use them, as well as a lot of autosport guys. The company is very switched on and keen to innovate and perfect their products.
I found the best way was to dunk it in a bucket of water and then let it drip dry for 20 minutes or so. A bit drier under clothing but still plenty of 'water charge'. I found as long as I kept out of full sun I could manage the heat quite well. I even got a bit 'hypothermic'
getting into late afternoon/dusk and had to take it off!
The only downside is that you tend to leave a damp patch on car seats!
Hope that helps,