There was an article in Good Old Boat where the guy built a water lift
muffler. To do what you are asking, the guy wrapped one layer of glass
and epoxy around a well-waxed pipe. After it cured, he slit the glass
and removed it from the pipe. A little epoxy and he glued it back
together again and continued with the lay-up.
Article 3367 May/June 2007. Pages 33 - 36.