Okay, long time Lurker, first time poster. I was hoping to Lurk longer so I wouldn't make my first post sound stupid, but I suddenly realized I won't be alive long enough to sound intelligent!
Long story short, you don't need to use a solid tapered plug in the mold to remove the core: I cast foundry furnaces quite often, and have a 26 guage core that is a slip form. I have a vice grip welded (behind the hinge pin) to the ends of the slip form.
Using the slip form core means that I end up with an (almost) perfectly smooth joint between the "doughnuts" in my furnace to keep from causing turbulence in the flame path. I cast "doughnuts" almost exactly like V1 in Chip Friedline's drawing above (minus the taper, of course).
FWIW, and I have no idea if this will help on an RMH but it seems in my pea brain that this would be an advantage in the Rocket to create more draft in the combustion chimney, I wrap 3/8" vinyl tubing on the outside of the slip form after I expand it, barber pole style, to create a vortex effect around the crucible. I know that there is no crucible in an RMH, but a vortex is a vortex, and creating more draft creates more draft...