I have been looking for sonotube locally around Tampa,FL and found a place called Tampa Tubes. The guy there was very helpful, and he said they have a 15" diameter tube which is 3/8" thick that they can cut to length. I asked what it was made of and he said "paper". Is this the sonotube stuff or something that is an acceptable substitution? I plan on trying a 15" dia by about 18" column for 54L volume. TIA.