Well assuming it's just some foam rubber strips with adhesive on one side, I
may have something for you. A while back I was replacing an exterior door for
my house and while I was at the building supply store (in this case Home
Depot) I found some (I believe) 3/4 in wide, 7 or 8 ft long strips of a foam
rubber-like substance for weatherproofing doors. I'm sure that with some
trimming this would work for the heater ducts as well.

Sean B.
'90 V'gon, "Happy Bus"