Using Windshield Sun Screens as Mylar?

i called my local homedepo today and asked if they carried mylar, he said no but they do carry sun screens (the silver things you put on your windshield to reduce heat) and said that was made from mylar. do you think this would be a good substitute? they are usualy very wrinkly and im not sure if that will cause heat spots.

I never heard of anybody using them before but i've seen them and thought to myself how they would be perfect! good luck!