I decided to try a synthetic rubber called Santoprene. It wasn't cheap, a 12" x 12" x 1/8" sheet cost over $20 from Home Depot online. I chose it for it's wide variation of uses in automotive manufacturing. It's supposed to be well suited for seals with long term ageing durability, resistance to extreme temps and flexibility at -60 C.

I dug the old seals out of the original vent doors and cut out the new seals using a paper template. The vent doors are two pieces riveted together and it's necessary to gently pry the outside edges enough to fit the seal between. I think it will work out great.

1958 Chevy Apache 32
283 small block V8