Acrylic is a plastic.

Clear and Transparent Acrylic Warning

This is a warning not to leave any transparent acrylics especially clear acrylics in direct sun. Always cover acrylics when you are in the sun so that you do not damage the ball or your property.
If you think you might burn something down then try some METALS. These are Chinese Excercise Balls or Bhaoding Balls or those metal balls or chimey balls.
These are a bit small in my hand. Right now I like to use 55mm to 63mm(2.2" to 2.5")