I went to Home Goods (stores like walmart should have them too) & bought a $9 welcome mat made from 1/4 closed cell foam (think that's what it's called). I did notice a small difference in the sound, It seems to have deepened the bass slightly & stopped the vibrating too. Well worth the 9 bucks.
1st, I used newspaper (cardboard is a little stiff to mold unless you dampen it & 8x10 paper is too small) to cut templates then traced that onto the foam & cut them out. I then used 3M spray adhesive to attach the foam. I bought 2 mats but only needed one because I traced the templates as close to eachother as possible.
I've taken the outer fairing off numerous times since & the foam is still in it's original spot.
Just be aware that you may need to re-route some wires/harness because it's tight to begin with in there (just the way I like it!)
I'm running 6.5' Hertz Hi-Energy's in the fairing & 6x9 Polks in the saddlebags powered by a 500W Rokker 4ch amp (as of right now)