Here are some close up pictures of the exhaust gasket. Garages that do a lot of exhaust work might have some spares or know how to get one. Take the Coke bottle with you to make sure it fits.
The gasket measurements are:
inside diameter- 2 9/16"
outside diameter- 3 1/16"
ring width- 7/32"
(Not pictured) With the Sharpie trace the outside diameter of the PVC onto the sheet metal. Cut out the circle with a jig saw. File and shape the circle on a grinding wheel or with a Dremel tool. The diameter should be 3 1/2".
Center the exhaust gasket on the sheet metal. Holding a mechanical pencil straight up and down, trace the inside of the gasket. There should be a small gap between the line and the gasket. Cut out the center of the sheet metal. I drilled some holes around the center then cut it out with a cutting wheel on the Dremel. File and smooth the edges with the Dremel. The inside diameter should be approximately 2 3/16".