Dude, I'm so glad to see you doing this because I actually am doing the same thing, except with different material!
One experiment I've done was took a black 5 gallon bucket, and spray painted the outside of it white.
Second experiment is to use thermal insulation tape and place it in strips around the entire 5 gallon bucket.
Both methods will aid in light reflection, as well as bucket insulation to keep water temps lower.We'll see which one turns out better, but it's awesome to see you trying a similar method! Let me know how it turns out!