Since the extra frames (made of beech) basically are copies of the bellow frames, they need to be thicker, to compensate for the thickness of the bellows leather's thickness. They also need to look fairly good.
In a shop for model airplane builders, I found some 0.4 mm plywood (!) – fine, but too thin, until I found a strip of rosewood veneer at home (had it for 25 years), 0.8 mm. Perfect, we'll use both.
Both were cut in strips, and one end was sanded/filed to a rough 60 degrees, so one would fit below its neighbour.
The plywood was then roughly trimmed and the rosewood strips glued on top of it. The rosewood was trimmed and then sanded exactly down to the frames.
Model airplane plywood - thin and flexible
Gluing plywood to the sides
The plywood needs trimming
Rosewood strips ready
Gluing the rosewood
Trimming the rosewood
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