My girls had a lot of trouble in the heat a couple of weeks ago, the heat came on pretty quickly and we weren't prepared. I found an old, huge saucer (like for under a houseplant), it's pretty large like 24 inches across, or maybe more. It's plastic and I fill it with water, they use it for a kind of swimming pool. They stand in it and drink from it as well. I refresh it a couple of times a day if I am around and it's very hot. They have their regular waterer, and I added another small bowl of water that I refresh at the same time as the pool. Also I have a large gallon container (from salad mix at the grocery store) and I put an electrolyte solution in there. They love that and drink quite a bit. Save-a -chick in the packets or I found a bottle of tablets that dissolve in water. I have noticed very little panting since I started with all the fluids all over the pen. It's almost like having random water here and there encourages them to drink more.
I did do frozen watermelon floating in the water, they loved that. I tried a 2 liter bottle, frozen with water and placed that in the pool, but it seemed to melt fast and they didn't find it amusing. Sometimes I run the fan on the run, just for more air flow, and I have been running the fan into the coop in the early evening so it cools off some before they head in for the night. My girls are afraid of the fan though, so I have to be sure to turn it off before they are wanting to go in.
Also I read that when it's super hot, it's best to leave them alone, not to bother them a lot as they get excited and that raises their temp. Same goes for treats (other than cold ones) because the process of digestion heats them up, and they get all worked up when they get treats. And no corn scratch when it's hot as corn makes them heat up. Good luck!