Originally Posted by frodobaggins
Another idea might be to use this faom you can buy in a spray can. I don't know the English name for it, but it is used to fill the wholes between the wall and the door frame when buidling houses, for example.
Use this foam and let it dry. You can cut it to the right size later and add a few layers of fiberglass.
Just my 2 cents, especially since I haven't tried it myself yet :-p