If you are sure it's the heat, perhaps a cheap box fan set in a good spot could keep the air moving enough around the area. I am in Oklahoma where it so hot the Devil thinks it's too hot LOL, anyway I have a large enclosed coop with outdoor access. The fans work wonders and have never lost birds to heat. sometimes all the ice bottle and mister stuff is just impractical, ventilation is the best cure. This whole raising chickens can get to be overwhelming at times, try and deal with it as best you can and then learn from it. Don't be so hard on yourself these things happen, keep in mind that it's important to realize to acclimate your birds as we have to acclimate to extremes as well, over compensating for weather extremes does more harm than good, I prefer to toughen my birds up for these kinds of extremes it has proven to be a wise choice, sure they can become uncomfortable but seem to get past it, that's the trick. Good luck in the future.