I use foylon (or alumaglo?) in my cabinet. It works great! It looks like duct tape in that it has all those little strings that reinforce the material in the middle, with a shiny reflective side and a non-reflective side. It reflects about the same as mylar and it has much better light dispersion than mylar. Only problem is it might be a little expensive (which isn't a problem in a small cabinet- don't need much) :thumbsup:

http://www.4hydroponics.com/grow_room/foylon.asp (thats a big roll... i got more than enough for my cab for like $15)