Ive heard many say "foil" is no good causes hot spots ect. I have a cfl grow and i need to make some sort of reflector for my lights ( 7 26w cfl's). From review Ive seen the "coke can" reflector mentioned many times. Seems like a great idea to me but whats the difference from this and regular "foil", there both aluminum, why does foil cause hot spots but "soda can" reflectors dont?