Can you come up with a 55 gal steel drum? If yes, lay it on it's side and cut off a section at the bottom of the drum to give you the taper when cut in half. Then you will want to weld extension on to give you maybe 2' more height, weld the open face (opposite the original barrel bottom) shut. Then cut a hole the size your arch needs . The (barrel) taper will easily support the insulated stack. Good luck.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.