I need to replace the seals on the flapper doors that control the entry of fresh air into the interior on a 1969 Chevelle non-air car. What material is being used to replace these seals?

The doors themselves are part of the Astro Ventilation 4" vent tubes (one on the drivers side and one on the passengers side) that duct fresh air to the ball vents on the dash. The opening and closing of the flapper doors are controlled from the upper pull-knobs on the kick panels. The original seals are sandwiched between the 2-piece door assembly and are riveted together. Surely someone else has had to replace these seals and has a suggestion. One of the seals is still intact enough to use as a pattern, but I do not know what material to use. Thanks!