Try to pull your intake air from the living space, which I assume is a comfortable temp.
Exhaust to outside is good; don't forget your carbon filter!
Insulating the attic is also important. I live in a Victorian with a walk-up attic that is insulated really poorly, with a big window that catches afternoon sun... it's blisteringly hot up there some days, and I fought high temps every time I tried to stink up my attic Do you have a way to reduce heat entering the attic, such as putting up blinds on any window you might have, and keeping the attic door closed in summer so heat from the house can't get up there? Then you'd need to think of how to get your cool intake air to the tent.
You're getting plenty of solid advice here! Good luck!