For driving around in a Slingshot in 100+F temps, what I've got and tried was the phase-change cooling vest by Techniche (their Techkewl Phase Change Cooling Vest). I got mine through Amazon:
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These work great! But, there's one gotcha (which probably won't apply in Palm Springs, more on that in a bit). This vest comes with a real nice insulated carry bag, so you can stow a vest with pre-cooled phase change inserts in the Slingshot -- and pull it out to wear when you need it. I used it on a 100+F day, sitting in stop & go traffic with a full face helmet on. I was nice & cool all through rush hour. I even tooled around town and re-fueled on the way home. This would have been absolutely intolerable without this cooling vest. Would a roof help? Yes that would help a lot too (but I currently don't have one).
The one gotcha was when I got home -- still nice & cool -- my shirt was all wet from the condensation on the phase change inserts. Granted, Techniche probably didn't anticipate this vest being used by someone sitting in a humid wind blast. But, if I need to use this vest to go to a meeting somewhere, I'll just pack an extra shirt. The phase change vest remained nice & dry on the outside. But, going to a meeting wearing something that looks like a bullet-proof vest would probably not go over too well...
The condensation would probably not be a problem in the high desert -- places like Palm Springs or Las Vegas. On the plus side, this condensation probably helps with the cooling a bit too.
There are a myriad of cooling vests on the market, some of which use plain water in their inserts. The plain water ice inserts are way too cold when you put them on and lose their cooling in just 30 minutes or so. The paraffin/glycol phase change medium used by Techniche feels cool -- not cold -- and keeps cooling for 2 hours or so. I keep the inserts in a tupperware container in the fridge or freezer so it's always ready to go.
There are cooling vests which use circulating water from a pump, using an ice chest reservoir. Those would probably last for more than 2 hours. But, for quick trips this Techkewl Phase Change Vest is very convenient. Just pack a dry shirt if you need to go to a meeting.
