Guv'nor, questions like .. why?.. for instance?
The bit I think you're asking about is a packing piece, 1 1/2 in. timber on the originals and 1/16 basswood on the models. All glued with bolts then going from the back through wooden solebar, steel flitch plate, said packing piece, and nickel silver axleguard. And the stepboard brackets where appropriate.
Not sure about stepboards yet. Whatever is used will need to stay straight and not wave about. Maybe timber, but perhaps not strong enough. I have some 2mm. tufnol sheet which may do. Or single sided PCB if it's thick enough (is it still obtainable?), copper side down and a quick solder job to the brackets.
Very open to ideas.