Welcome! My wife and I made a rock-like background using Great Stuff expanding foam, acrylic paint, and clear silicone 1. Those are all animal safe, at least from what I researched. I also would expect them to do well in 100 degree temperatures.
We sprayed the foam (we used the Pond and Stone variety because we wanted to the black color) onto the enclosure and waited for it to cure.
We then carved it into the shapes that we wanted using a serrated knife.
Then, we painted it with acrylic paint in various colors to make it look closer to natural rock. Make sure to use a dry-brush technique.
We sealed it with some clear silicone because our vivarium will be humid. One tutorial we read said to mix the silicone with some powdered sugar to not make it glossy, so we did that too.
This is the result:
Also, I will note that another forum user, Veronica, made a post about her first build with Great Stuff foam and provided some good feedback for me on this thread:
http://www.ssnakess.com/forums/enclo...oam-build.html
A few other links that might help/inspire:
https://www.youtube.com/watch?v=HvzZQdrMU0Q
https://www.youtube.com/watch?v=NA22A5T6xl0
https://www.youtube.com/watch?v=Mpko5rxM9pI
https://www.wikihow.com/Make-Fake-Rocks-for-Your-Pond
Good luck!