do u mean like in the trunklid bracing or under the rear dash those small holes where u cant put normal deadener ? - use expanding foam like a product named GREAT STUFF it works really good ......
I would guess you should use sheet metal if you have holes in your trunk ;)
I did the expanding foam trick on my last car, messy as hell but it worked great, just make sure to leave PLENTY of room for the excess to escape, or it may extert pressure on the exterior panel of the car and bend it/crack the paint.