If you cut your own gasket from FLAPS cork gasket material ...
I found it best to draw your gasket on paper. Attach some blue painters tape to the gasket material. Then attach the paper pattern to the blue tape with spray adhesive. The tape is a lot easier to remove from the cork than the spray adhesive.
I cut the cork with a single edge razor blade.
Replace the gasket anytime you empty the bowl. See lower right corner of this
"Cadillac Serviceman." It will turn into a cornflake if it gets dry, and it won't return to round and flat when it gets wet again.
Classic Truck Parts, Jim Carter, Chevs of the 40s, and The Filling Station sell a gasket for glass bowl pumps.