A possible source for flexible ducting that resembles the type used in CMPs.
http://www.aircraftspruce.ca/pdf/201...l/Cat13111.pdf Aircraft Spruce also operates a US site at aircraftspruce.com I suspect shipping to many parts of the world will be more than the price of the ducting. Confirm on your truck but a quick and dirty measurement on one of Bob's yardbirds suggests 1 foot of 2-1/2" ID should be enough for a truck with a bit to spare. US and Canadian pricing for that is around $10-20 for that quantity including postage/shipping.
Other sources might be your local airport (but go to the Cessna side, not the Boeing/Airbus side) or anyone in your area who's into homebuilt or experimental aircraft. This part of the Experimental Aviation Association's website might help you find someone in your area.
http://www.eaa.org/en/eaa/eaa-chapte...an-eaa-chapter They might be as curious about your truck as you are about the planes...