1. turn a cone from steel
2. form a light card cone to fit
3. cut out a reasonable thickness ally to the pattern
4. get a mate to weld it (hence the need for over thickness sheet)
5. file internal and external welds flush
6. refit ally cone snugly back on steel mandrel (point 1)
7. turn ally cone to required thickness
8. using a thin blade part off cone into required number of sections.
the cones concertina into the required horn when stacked.
Super solution & he's already sold one on ebay!
seems to me that points 1 to 7 would solve this exhaust cone scarcity very nicely, thank you.