Static cling window film works fine. Just slap it on the the inside of your windshield when not needed for the mirror.
https://www.autozone.com/sun-heat-pr...on/window-tint
When cutting it out for the mirror make sure you make it at least 1/2" longer on the width side. Then cut that down leaving a 1/2x1/2" tab protruding. Place Scotch tape on the tab backside so it's easy to pull off the windshield.