My second 1965 Dodge Dart.

Johnny Dart said

Google firewall insulation pad 1965 Dart. They are available from different vendor's. They have it for AC cars, and non AC cars. Should be about $40 for non AC cars.

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Do you mean buy firewall insulation and rip off the insulation, and cut a piece of a card board and use for the heater box ?

By the way, the cardboard is supposed to be 1/16 inch thick.