I put a cool block from the freezer in the coop covered with a cloth. works instantly. Bigger the coop, more blocks. A bag of ice would work the same but wouldn't last so long,
The hot weather changed suddenly on the day i put out my two four week old chicks. I put in an old fashioned hot water bottle under a bath mat and cover with hay. They were still warm in the morning. A wheat bag might do the same.
I had the same problem. husband increased heat by switching on brooder lamp accidentally. Hatching day water was dried out. horrible, i managed to get two out four eggs out by hand when I realised what was happening.