How do I keep my phone cool while taking photos in the sun?

I take headstone photos in cemeteries using an extender and a remote clicker. First the screen gets dark, and I can't see my subject. Then my iPhone 15 Pro overheats, and I get the message to let it cool down. I have had a similar problem with all previous models but the 15 seems to be worse. Anytime the outside tempature gets over 75 degrees I have a problem. I take the case off and try to keep it in the shade as much as possible. A large cemetery could take 4-5 hours. I get the cool down message in 15 minutes. What can I do to extend the time I am taking photos?