There was a guy on here who built a small grow box and used one of those silver reflective car window shades that he cut to fit the inside of the box, looked nice to. Said he picked one up pretty cheap. Dont know how well it would work though.
Tin foil seems like a good idea, but just wont reflect like mylar. Tin foil was not made to be very reflective, where mylar was intended to reflect light. Go to a party store, ask for some mylar wrapping paper. It will be very thin, and cheaper than "hydro store mylar", but the thicker,4mil mylar works so much better. 4mil will last longer, and can be scubbed clean.
Less than that. The cheapest way to go is flat bright white paint, or step up to mylar. And if you do mylar I would not monkey around with space blankets or wrapping paper, buy the real stuff, horticulture-grade, at $20-$25 for a roll.
yeah i saw that post. i was thinking about doing that but i would have to drive about 45 minutes to glenwood springs to get something like this..the grow store is there too so i would just buy mylar haha