I don't think you are going to notice a huge difference if you try using something to fill the space behind the speaker mount unless it is a sizable gab between the speaker and the door. Keep in mind the door has many other openings in it in which air can escape. You may want to consider going to best buy or looking for a place online (probably get a cheaper price) and buying the Dynamat that comes in two square sized pieces that are just big enough to fit behind your speaker mount. This is an easy job to do and saves you money from having to buy a whole roll of Dynamat.
