Hi!
I originally used it straight out of the long plastic roll it came from (from Home Depot)without doing anything else. It sounded like a muffled veil was over the woofer. I places the cut sheets on the three sides as Wayne suggested previously. They looked about 3.5 - 4 (I didn't measure the thickness) inches in thickness. I then split (literally tore apart) the cut fiberglass pieces to 1/2 of the original thickness and then put them back in. Much better...they are now 1 to 1-1.5 inches in thickness and sound quite nice. I also added some additonal bracing to both cabinets although now I think it probably wasn't necessary and might have over done it, but they sound good and that's what's matters!!Room acoustics, room size and placement will obviously play a major role in how they sound and what I did may not translate well for your room /space. As always.... your mileage may vary(ymmv)!!
Cheers!
Frank Mena