On the outside, I did the same but 2 layers. It was a lot of work filling, sanding etc, but the end result is very smooth and pretty. I wanted a waterproof, rigid tube with good thermal properties, and I believe I've succeeded. Next time, I would carefully butt-joint the glass tapes on the outer layer, and use two layers so that the joints end up staggered on the two layers.
I've thought about a foam core for years, and saw a beauty under construction at StarFest (Ontario, Canada) in '93. The foam I would prefer is Airex, used in composite canoes, and I believe, the premium product. A sonotube mold, to be stripped out later, makes the most sense to me. It is necessary to put carefully made wooden discs inside to make it truly round. I've been wondering about using one of those camping mattress foam pads as a core. They are about the right thickness, and fairly tough. The tube wall should be at least 1/50th of the diameter, for strength.
I use boat-builder's epoxy: West System, it's wonderful and reliable.
I realize that the goal of a 15" composite tube is to save weight, and I think it will work well. It WILL be an awful lot of work.
Larry Manuel, Calgary, AB.