Excellent work and ambitious project !
you may consider using sprue to scratch your air ducts, if you can find some at the right diameter it could be perfect.
If they are too small you can wrap them in paper stripes and put a coat of putty above, with some sanding it should be smooth and at the corret diameter.
Other than that, just find some plastic rod at the right diameter and bend it...
I also use some electrical wire, as they are several diameter available and some are semi-rigid.
I also (re)build exhaust with the same technic.