Update
 

Got the Dorman/Help! part (#96003) and managed to salvage the soft rubber (not cardboard!) cone off of the old duct.

They don't q-u-i-t-e fit, so it's going to take some improvisation.

At this point, I'm thinking that I should basically sew the duct to the cone and backfill the gap with silicon caulking. Using steel wire, I could loosely sew them together in three places, then tighten the steel wire once the silicon has set. In order to clear, the ends of the wires that will get twisted together will be inside the duct, and not outside the cone.

Salvaging the other end presents more problems than I think I can deal with. Rather than use the existing latch/clamp, I think I'll go with a regular hose clamp. The problem is that there's no really effective way to attach the old clamp to the new duct.

It will be interesting to see how well this Dorman/Help! tube its onto the heater.

Are there no salvage yards in your area?

Zip ties?
https://www.ebay.com/itm/Lot-1-1967-...hY-lt-&vxp=mtr

Nearest good yard for trucks is about 25 miles away... there's another that's about 35 off in the other direction.

I thought about zip ties, decided I'd rather go with a stainless steel hose clamp.

Think I'd rather use some kind of wire clamp made out of spring steel. A piece of regular wire wound tight if I had to. Something not so wide as a hose clamp since the duct is spiral wound.

There's an idea... and it wouldn't be the first time I've repaired my truck with some bailing wire... it's just that bailing wire is usually a temporary solution.