What's wrong in glueing just a FEW layers of cloth - one inside, one outside the cardboard tube, and maybe strengthen few critical areas with another layer or two - around focusser/spider/finder etc. It will be almost as stiff as solid fiberglass, but several times cheaper and lighter. This way you can use high strength carbon fibre and get even lighter and stronger tube than solid fiberglass ! (with almost zero expansion coefficient and exceptional resistance to dewing as a bonus)
Am I missing something here ?
Bratislav