make sure that you COMPLETELY cover the backside of the sub with it, cover the motor, and basket, and if you can leave a little overhang to seal it to the back side of the box that will help out a ton.
that keeps anything internal from vibrating, and keeps the waves from leaking out the backside of it, like the dynamat diagram shows (like with the trunk of the car, but it is equally important inside the enclosure as well.)