Got a 1970 Skylark and am wondering how the defrost ducts seal to the heater box. Peeked inside dash while radio was out being serviced and saw it wasnt attached. I found a screw and reattached it to the heater box. But using the factory hole I got a gap all around where the 2 meet on top of that both are hard surfaces and there is not indication of a slip fit. Is there supposed to be a seal/gasket between these 2? If so where can I get or how can I reproduce the type of seal that should be there?