The larger majority of the folks here have been in the same boat as you are now. Here's a few ideas. Not sure of what all you need to do, but,...
An EZ up canopy will keep the direct sunshine off of you, as will a $99 10X20 canopy from Northern Tool
With some cover, a cage fan helps out a lot.
When it's excessive hot, a cold towel around your neck, and or on your head will keep your body temp down and allow you to continue on.
Again, not knowing much about you or your schedule, one thing I did when I was having to do a lot to my Jeep, I worked late hours and even all night, when it was cooler, and left it alone and slept during the day.
I was also fortunate enough to have some friends who allowed me into their shop to use to do some projects at different times. I did my best to not over stay my welcome, always cleaned up my messes, in other words, I was a good visitor.
Just some ideas