A customer of mine came in today with a piece of PVC pipe that was 16" in diameter. Maybe some of you have seen PVC this big, but I never have. The walls were 1/2" thick and he said that it also came in larger sizes. Price was about $10/ft. Any reason why this wouldn't work great as a sub tube. It would be slightly heavier than a sonotube but finishing would be easier - the smooth surface would just have to be painted. I don't know what the resonant freqency of this stuff is though. If he has some sections left over form the job I think I might try making a sub or two.