When you buy a replacement impeller for the spares kit, it will probably have a new gasket for the pump end plate in the package. If you lose this, damage it, or are simply using a second-hand impeller you prudently kept against emergencies, you’ll need some sort of replacement. Success might be achieved with gasket goo, without which no yacht should go to sea. Failing this, lay the end plate on a piece of heavy paper or thin cardboard and gently tap all around it with a hammer. Lift off the plate and you’ll find a perfect impression. Cut this out with the galley scissors and it usually works as well as a bought one.