Shade. That's the key, I think. Site your coop in the shadiest part of your property, especially with respect to the afternoon sun. I'm in North Texas, and I stupidly put my open coop with wire walls right smack dab where it gets the afternoon sun. Even with all that ventilation, heat builds up under the roof and makes it hotter than you know what by the time the sun sets. It takes hours for the coop to cool down even after dark.
My new coop is a conventional, closed sided one, but it's in the shady part of our yard, it's insulated, and the roof has radiant roof sheathing. The inside of this coop gets no hotter than the ambient outdoor temperature in the shade, which is plenty hot enough but the best that I can do for our climate.