Custom vent for through-the-wall AC unit to another room

Hello, I just replaced a very old (30? years) through-the-wall AC unit with a new one, the only one that snugly fits into the existing sleeve. The old one was meant to service two bedrooms using a custom built duct that was built around the old unit. We tore it apart to install the new unit, and now I have to think of how to get the air from the new unit into the existing duct.



old setup:



I'm considering:

1. Some custom carpentry
2. Some kind of hose/sleeve like dryers have
3. A 3d printed part

Any ideas? Thanks!