There is a post still on the first page of this forum discussing the use of mirrors as reflectors with artificial lights. The same principles apply if you wish to reflect additional sunlight onto your plants.

When trying to reflect light and well it should be mirrors do reflect but diffuse light also.Mylar and white plastic(black on 1 side white on other)or flat white latex paint on whatever material you want to coat has been used by many growers for years.I was sending you a link but I believe it would be against site policy but you can get Mylar and Black and White Poly from the internet at Hydroponic Stores.