A piece of board can be made into an efficient reflector by covering it with aluminum foil. The foil will work better if it has been scrunched up first and then partially smoothed out. This leaves a stippled reflective surface that will reflect the light more evenly. Keep in mind that the quality of this kind of fill light will still be more directional than that provided by a white surface.
From The Ultimate Field Guide to Photography (National Geographic, 2006, $25)