Haven't actually used those, but have felted my tweeters with what I feel to be good results. You can go to any fabric store and buy felt, cut to size with an appropriate opening and then tape to your speaker cabinet with double sided scotch tape. I have built mine up using 3 layers of felt (about 1/4" thick maybe?), but you could experiment with more or less, and the felt and tape shouldn't cost you more than $5. Plus you get to choose the color of the felt.
For what it's worth, Ric Shultz at EVS mentions felting on his website as an excellent tweak for virtually all speakers.
For what it's worth, Ric Shultz at EVS mentions felting on his website as an excellent tweak for virtually all speakers.