We use the add-on sunscreen from an auto parts store. The kind used for automobile windshields.
Apply a 1" strip across the top of the helmet visor... on the outside of the visor.
It is not low enough to view through unless you tip your head down, but it cuts the glare from the sun most of the day (not late evenings).
Easy to apply, easy to get off, ok to have some lack of smoothness since you don't see thru it.
Scoot safely,