I'd suggest if you were to use foil that lighting foil would be a better idea than reflective aluminum. It's a matte foil of a sort that's designed for use on theatrical and studio lighting equipment and is designed not to heat up as quickly, if at all. Since the idea is to cool it off the external heat from the room will not affect the foil, nor will any heat coming off of the side of the computer.
Kind of silly for a laptop that's presumably portable, though ;P
I have a spare fan in the closet, might try something similar that draws some air from the air conditioner and pumps it to the back of the computer. That thing gets hot :[