You can find this damping materials around. Another equivalent is rock wool (instead of glass). Similar properties of different materials are available like this one, COPOPREN by Recticel of 2cm thickness, a sound barrier/foam available from "Leroy Merlin" that I am going to use this year. Pay attention that the speaker was tuned already by Wayne with the first fiberglass insulation solution. I like also another (to) fluffy material used in sofas and pillows that is Polyester Fiber, available at "Mosqueteiros", etc.(many other places).