Is your house raised? If so can they go under the house? Ours isn't but mum and dads dog used to love that.
Our rottie in particular feels the heat, but she loves the clam shell. She will jump in and out all day. Is there dirt somewhere they can dig/lay on? A lot of dogs will keep themselves cool by digging down a little and lying in the hole, our girl does this also (she has her own part of the yard that she does it in).