Actual furnace fans aren't that loud, or they wouldn't be tolerated in homes. Why don't you try to find a trashed furnace and salvage the fan out of it to see how it does? It can't be worse than the plastic box fans, and can be ducted way easier.
Furnace fans can probably be found by asking around at some local shops that service/install them. They're almost always replacing furnaces when people upgrade them, but the fan/blower units are probably still good.
Here's a link to a Canadian woodworker that made video of exactly what I was thinking.
https://youtu.be/lLfxKhJUgbk