The piece of neoprene you are talking about is  about 5mm neoprene wet suit type with cloth to give strength. It is shaped like an egg with a  shaped cut to seal on the anchor chain. It should extend out over the pipe to seal it by about 3mm all round with the exception of a small extra nipple where the light chain secures it with 2 small washers screw and nut and again further for the egg shape is the ease of handling when  sealing down the cover. Very easy to manufacture. ,  

Regards Joe