It's called a focusing cloth. I use an old one made and sold by Zone VI - Calumet may still sell them. There are other commercially made ones.
Some people go to the fabric store and buy the cloth that's made to go on the back side of drapes - it's called blackout cloth or something like that. It's supposed to be fairly lightweight and light proof. They sew their own.
I don't know of anything that helps with the heat, though. I'm in Florida, so I know what you mean.
Depending on what camera you have, there are attachments that fit the over the groundglass forming a hood with an eyepiece. I have one that came with my old Omega (Toyo) 4x5. It's certainly cooler, but I just don't like looking at the ground glass with one eye.
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