Perhaps harmonic resonance would have been more correct, I dont know. My understanding of this stuff is very rudimentary. Anyway I want to maintain the volume of the cup and believe a very thin layer of cork rather than felt may take up less volume than the felt. I also plan on using some duct tape in there to help dampen things a bit. I'll know after I try it, how it works. I'm replacing the blue tak with plasticine to improve the baffle damping. Right now I'm actually pretty happy with it the way it is. Which is just blue tack in the baffles and felt in those pockets in the cup. It has this sort of caverny, echoey, platicky inorganic sound that's not quite right but my understanding is that can be tuned out via damping.
that is exactly what the acoustic foam will get rid of. Wait for me to send you some.