millions of people do....
They sell them for cars.....shades out of that stuff... I have one in the RV that I was gifted, printed with the crew of the Millennium Falcon on it
Buying it in bulk, it's called reflectix. I ordered a roll from amazon, and made custom fit shades for every window in the RV. I find they stay up on their own fairly well tucked into the window frame...and for the house windows I double that up by pulling the shade down behind it to help hold in place.
I do it for storage primarily, but occasionally will use it when RVing to keep the heat out....
I even made some to fit the skylight and roof vents, but later replaced the roof vent shades with those pillow things they sell....
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