Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> Rather than fool with trying fill innertubes with relatively expensive foam in a can.. Buy big blocks of expanded polystyrene (trade name styrofoam), and use either a hot wire (best results) or an electric carving knife and a template to make a toroidal foam block. Sand, "bondo", cover with your preferred conductive surface.