I have the same problem and I have been spraying their chicken yard to help keep it cool. I also put in several plastic containers with water for them to stand in. Chickens cool through their feet I have been told.
I bought an inexpensive bag of frozen corn as a treat.
So far so good but...
My Cuckoo Maran is 70 days old and I think it’s a Rooster. Does anyone adopt these chickens? I live in an area where I can have Hens only. If I can’t find a good home I will have to have Bertha put down by my vet.