Thanks for all of the suggestions. I went down to the Napa store this morning and the parts guy brought out a drawer with a bunch of different types of
oil plug gaskets. They had the Dormand, but the diameter of the copper ring was too large to be effective.
I ended up finding a rubber gasket (Napa PN 704-1964) that fits tightly around the
oil plug with a perfect diameter to accommodate the copper ring and the pan surface. I put the rubber gasket on first, then the copper ring, the the
oil plug.
Seems to be working so far.