If you are carpeting the box, I wouldn't even bother with the gasket. Some subwoofers have gaskets from the factory on the underside. As for the poyfill stuffing, you only need that when the box is smaller than what is required. Of course, there is a limitation to that rule. For example, if the box you have/made is 1.25ft^ and the sub is calling for a 2.5ft^.....all the stuffing you can pack into the box isn't helping. You will just need to get/make another box to the appropriate size.