"6 Giorni" Looking for the rubber flap on the top of the 6 Giorni tank it could be called sozius seat

Hi all,

im from melbourne Australia and im restoring a 53 6 Giorni replica. im looking for that rubber flap on top of the tank lid. I dont even know what its called to search for it. anyone have any ideas where I can find it or will I just have to make it myself.

thanks