Putting foam in small enclosure like a jeep's roll bar speaker thing won't do much at all. The idea is you are fooling the speaker to think its in a larger area. If I'm not mistaken it will just lower the bass the speaker puts out giving it a more crisp sound. It generally shouldn't be packed because then you will be closing up the area. Seeing as its a small area anyway its hard not to pack it. Normally in sub boxes the material hangs off the walls or is laid in the bottom not compressing the foam. I know they make thin material that would work for the jeep's roll bar speakers but you are better off buying a higher quality speaker if you are worried about the sound. Foam is cheap enough to try and easy enough to remove if doesn't work out for you.