On a locomotive the “petticoat”, more usually called “petticoat pipe”, is the bell-shaped inlet to the chimney.

If you use metal for the crinolines you will reduce the cladding’s effectiveness a bit. PTFE, SRBF or similar may work but determine the candidate material’s maximum working temperature first.

Of full-size boilers the hoops seem to be supported on spacers, and you might examine following suit, with PTFE, SRBP or hardwood spacers below a thin brass band on which to support the cladding sheet and its external closing bands.

I don’t know the specific design but usually the outer sheet is wrapped right round the boiler with a single longitudinal joint along the underside. It is held by narrow bands sited over the crinolines and the rim of the smokebox; each band closed by a nut and screw or a screw into a tapped bush on the band, also under the boiler. This closely replicates full-size construction.

You could split it along the horizontal centre-lines as you suggest but it would look odd, unless it was a feature of the prototype locomotive.

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The outer sheet, or wrap, is often of brass on models although it’s not the easiest metal to paint (using suitable etching primer). It can be of very thin mild-steel sheet though, preferably tinned or galvanised but certainly painted inside and out. It should not become wet inside in service, though that’s a risk if you use a safety-valve as a cold filling-plug.

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What does the designer show for fitting the outer dome, which is usually a cover over the actual steam-dome? Sometimes it is held by a small, discreet, central screw into a tapped, blind hole in the dome itself. (Not a through-hole. You don’t want a potential steam leak there.)

Small details like hand-rail stanchions and pipe-clips are secured to the wrapping sheet, perhaps also an underlying crinoline, according to the individual engine.