There is a discussion on YORF about the possibility of moulding rocket parts using Papier-Mâché. The thread provides some references, including a link to a paper clay product that is available at Michaels.
I have tried making a small teardrop airframe using strips of newspaper and paste. It started looking OK but as it fully dried the surface warped. I ended up using it on the tube-launched Stink Bomb (Named because its resemblance to some stinky item. Which substance is left to your imagination.)
I think I'll try to make some sort of cheap 29mm cone for a possible rack rocket project. I may grab some of that clay and will experiment with my own concoctions. Modge-Podge, wood glue, epoxy? But where would I get an abundant source of paper fiber?