Bill wrote:

I am looking for low cost (as low as possible) acrylic sheets or Lexan
to cut into size to put over the inside of my large 76" X 68" deep
casement windows for less heat loss. Any steers on where to find this
stuff or something else that would work? Must be as transparent as
acrylic (.080").

Whip up some sort of simple frame, and use the hair dryer shrink to fit
window film over the resulting window.