My wife and I have used the foldable things they make for you to put in your car windshield to keep the sun off the seats as reflectors. I believe it worked pretty well, and you can get them for a couple of bucks. The one we have has a silver side, but it is very wrinkely, so i figured it was sort of self diffusing.
EDIT: This thing. https://www.amazon.com/Windshield-Sun.../dp/B001DZWXTW