a picture is worth a thousand words...
here's the stuff that i used. it's called FoilSkin by a company called Bakor. found it at Home Depot for, i think, about $14

for a roll 23 cm by 3 meters. you can find it in the roofing section. it has a very adhesive backing and it doesn't smell all that bad like other products. it's moldable (that a word?) and waterproof. which it should be as it's really used to seal air and water leaks around corners, pipes and vents.
props to machy for the idea. i knew it was out there but didn't go hunting for it until i saw pics of his install.
-rayray