Jim,
I found some expandable aluminum duct in my local auto parts store. It
was 50mm id, but easily squeezed down to be clamped on to the 1 3/4" stub on
the main duct. The part was sold for air intake use (eg. warm air intake off
the manifold) length about 10" expandable to twice that, $2.99.
Robin Leigh, 65TW.