I need to start planning my DC duct layout pretty soon. I have a couple of problem areas that I'd like to work out first with something real cheap to see if I can get bends minimized, etc. before I actually spend money on the final ducting. I was originally thinking paper or cardboard tubes, but cant seem to find anything in 6". It's also not flexible but I could work around that. Anybody have any suggestions for some kind of 6" tubing that's cheap and readily available that I can use for working out some duct run kinks? Flexible would be nice. Something I could cut with a utility knife hopefully. ??