Mesh kevlar long sleeve shirt, don't want road rash all over your arms...
Wear a T shirt, soak it in cold water before suiting up. Fill a large (2-3L) hydration bladder full of ice with some cold water. Put it inside a mesh MC jacket. Joe Rocket Phoenix or Ion model jackets are examples of the open mesh I am talking about.
The ice will melt and the evaporation will help cool you if you are not in stop and go traffic. Then the ice pack will melt slowly enough to give you cooling to your core. You need to use the bladder outside its typical pack.
When it melts stop at a convienence store for something to drink and refill your hydration bladder with ice before you get your drink. When ready refill your cup with cold water and do the wet t-shirt thing again.
May want to pack a small kit to wash up a bit in the restroom when you arrive at your destination, along with clothes suitable for that location. You can do your Clark Kent impression...
Or, as a friend in AZ would say, I couldn't carry on the bike what we were bringing with us today, a carful of cold air.
You will adjust