There might be a few other places to find tubes. Concrete places usually have "sonotubes" or tubular forms in various diameters and can be cut to length. Also another place to look are carpet/vinyl stores (including remnant). These stores sometimes have large tubes for vinyl. I have found some up to 14" in diameter and 10 feet long. If you find some and they let you have it, might want to bring a saw to cut to a rough length you need. Anyway, a few ideas to ponder. Here is a webpage of my 10" f/5 I just recently finished. This is a 50" focal length in a 48" tube.

http://home.kendra.com/comet61/TELESCOPE/10Dobs/My10Dob.htm