A quadzillion people have used and will continue to use 'regular tinfoil'. Just put it's dullest side out and it works fine. I use it and all is well. Shiny side out is supposed to create hot spots by reflection and supposedly could burn the plant. I don't think that's the case with fluorescents. Maybe with hid's though since they're so much more powerful.

Yes, mylar or other surfaces may work better, but I'd like to see some results that prove it. Actually prove it. Not just heresay.