Reading how you plan to go about making this horn, I offer the following (this first one you probably aready know):
Make the outside of the mold in quarters or thirds and hold it together using fabric straps (not bolted flanges)... If you make it in halves the mold will split due to expansion of the casting material when the chemical reaction causes it to heat up... The straps will flex just enough to allow expansion.
Also, make the rear chamber plug a separate part from the main "volcano" form... This will make the whole thing a lot easier to pop out of the mold, as you will not be forced to unmold a multi-hundred-pound part that will bind due to the double draft conditions (tested!!!!). It will also allow you to fit different diameter rear chamber plugs later on.
Also, when using a template to form the plaster, the key is to load the plaster on and turn or cut it while it is still very very liquid... Just keep splashing it on and turning (really a two-man job). Obviously, with a horn this big you will want to rotate the template, and leave the assembly stationary... The challenge will be to make sure the rotation template is RIGIDLY mounted at both top and bottom, and is held in a RIGID frame.
I use polyester resin to seal the plaster surface, then floor wax (the real stuff... the kind intended for hardwood floors) as a mold release... it is way cheaper than buying "Mold Release Wax"... I used to complete the process with a coat of PVA (poly-vinyl-acetate, a commercial mold release agent), but this is just a waste of money... 2 or 3 heavy coats of floor wax does the trick.
Get on a high-carbohydrate diet one month before starting work!
I look forward to seeing the images!
How to short-circuit evolution: Enshrine mediocrity.