Get an old speaker( same diameter as the subwoofer, I think it is 6" or 6.5) and peel off the spongy part slowly, using your nails...(takes about 10 minutes.)
Get a bottle of Rubber Cement from Hobby Lobby ($2.99)
rip off the broken one from the subwoofer and replace it with the new one.
wait for an hour or two and it is ready to go...
subwoofer sounds as good as a new one.