You may have seen this stuff. It is a dense black foam material kind of like the white plastic foam you find in shipping boxes. Not sponge rubber but denser. As an example it is used to pad the detachable Wave Magnet on a Zenith Trans-Oceanic Royal 1000. it is affixed to the top part of the chassis above the phono input, the Wave Magnet sits on it. I would show you an example but it has completely rotted away. I am sure they make it but I have not figured out a search term to get me there yet. Any ideas?