i was messing around with this after i put on a "free flow" muffler to try and reduce some low end resonance. i used sound absorbing padding that is used for laminate flooring. i covered the spare tire well and added a layer under the trunk mat and behind the gas tank. i also placed some under the rear seat. i thought i could hear an appreciable difference - not enough for what i wanted though, because i wound up reverting to a stock muffler. the stuff i have is 1/4" of dense rubber material with a fabric backing made from recycled tires. they usually sell this as an "upgrade" to the regular padding that they install.
1993 300e-2.8 - gone now <sigh>
"Do not adjust your mind, it's reality that's malfunctioning"