I've been looking at that enclosure and driver (the kit actually). I can't get really good numbers on the box, but if it is 3 ft^3, then WinISD predicts a Qtc of 0.85 To my mind, that's a little boomy for music, fine for theater. I think the reason is that they also use the same box for their 15" woofer, so it's a compromise.It looks like you need 4.7 ft^3 to get close to 0.7. On that basis I'd go with a fairly dense level of Fiberglass - to my understanding it will add the maximum amount of damping, hence "volume", and I think you'll want it.
FWIW