If you want the plastic window to last, don't put the top up or down below 70-75° F, if its colder use a hair dryer on it before folding it. Second, make sure to "chop" the top, especially when it is cold. The temperature affects how soft or hard the plastic is, if you don't "chop" it so it folds smoothly, it can fold with sharp angles which makes it more prone to cracking.
I also put some fleece in there but that is more to prevent scratches. My plastic window is still looking good after 22 years.
