If air-wound coils don't do what is required then a gapped ferrite core will help and allow for a lower input power. Brass' good conductivity is a problem for inductive heating. The trick for this and uniformity is to have a coil that concentrates the flux on the shoulders. The neck is easier to heat as it has shorter current paths, a lower mass of material than the shoulders and walls, and heat can only conduct out of the neck in one direction. Necks will keep up temperature-wise with the shoulders with much less input. Then the field also needs to drop away rapidly in the other direction to minimize heating in the case walls so the heating down towards the head only gets there via conduction.
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