if you don't mind using a little fiberglass, you could use anything (of the right diameter) as a mold, cover it with release agent, and wrap it with fiberglass.
for a rigid, very round, and cheap idea for a possible mold, go to a construction site and look for a scrap length of 8 or 10 inch PVC sewer pipe----those guys are always throwing away the odd lengths of cut-off stubs
after you wipe off the mud, it's very round, usually quite smooth, very sturdy-----we used some of that stuff for a different school project and it was way solid (and a pretty green color too)