this is what i do with my "barrel extenshions. i learned this from a friend that was into remodeling, he said this is how the sound proof rooms. in some cases.
I took a pvc tube, about 1/4 inch. in diameter, and cut it to the same length as the inside of the tube, i then drilled a ton of holes into that 1/4inch pvc, i wrapped it in a single laywer of cotton. then i took the pvc and placed it in the can, then filled around it with Down feathers, out of an old jacket. make sure not to pack it in too tight. and whala. It makes a pretty decent deference makes the POP more of a thud.
but i guess foam might work pretty good.