Foam-Art-Board (black) and Lexan (clear plastic) are the preferred ducting materials.
Michael's Arts and Crafts carries 2'x3' panels of the foam board, pricier than you'll find it at Target, where a single panel may be less than $2. You can -- with the right glue from any hobby store (expensive s***, though) make ducts using combinations of the two materials.
The art board doesn't enhance the LED-illuminated case interior, but it is also a good prototyping material, easy to bend and cut, makes rigid structures with the right glue (and not necessarily the expensive stuff), deadens sound, and non-conductive. Lexan is prettier, takes more tedium to cut and shape, also non-conductive, heavier than the featherweight foam-board, and needs "Poly-Zap" Lexan glue which goes for $5+ a bottle.
Starting with the foam-board, get yourself a nice straight metal ruler, an Xacto knife and a box of blades for it.