[jumpsoffrock]

I have also considered installing rigid ducting. My best advice would be to go to junkyards and piece together some kind of duct work from other cars.
Problem?
Most vehicles with rigid ducts had them perfectly designed for that application.

Using PVC pipe from hardware store? Sounds like a clunky nightmare.
Not to mentioned being laughed off the face of the planet the first time some guy sees what you did.

Some Youtube guy who owns an automotive shop said that an old man came into his ship needing electrical work on his 20 year old car. The Youtuber found the car had been rewired with Romex. The old man said rats ate the original wire so he redid it himself...


Nonetheless...
I have read that flexible line slows the speed of the air, and doesn't hold the air temperature as well, I'm sure it's not the best, but I bet it's good enough.