if you want to go with pre-formed plastic tubing why don't you go with the grey plastic electrical conduit. It is made of thermosetting plastic so it can be softened by heating and will harden once cooled. You can buy pre made bends as well in 90, and 45 deg. and I believe it has a heat resistance between 1 and 2 hundred degrees. they are available in 2", 3", 4" and so on. It is also inexpensive. I believe about $18 for an 8' section of 3", and the bends are just a few $ each as well.
The conduits are slip together and can be made air tight by lightly sanding the mating surfaces and then wet assembling them with RTV silicone.
Or you could really chince out an use the flexible aluminum vent ducting, it is easy and quick, but it shows, but if it works who cares, right?
