The firebox is a bit trickier. A two-layer backhead is built up first leaving an edge the thickness of the brass around the outside. The wrapper fits into this, more by luck than judgement as it's all done my eye.
Then the instructions call for a length of 2mm wide scrap etch to be soldered around the back edge and polished. Calls to use scrap material for something as important as this are rare. You are asked for the odd packing piece or filler, but a visible part?
Good job there is a suitable part on the fret and to be fair, it's better than wasting metal.