Mirrors absorb a lot of light, you'd be better off with flat white paint, white sheet plastic, or mylar.
Thick mylar is good, space blankets not so much. Many use them because they are made of mylar, but they are super thin and semi-transparent -- hold one up to the light and you can see right through it. Light passing through is, by definition, light not reflected. Stick to "real" mylar or flat bright white paint.