Spent a little time this afternoon cutting lining stock out of a thin willow board, then here thinning it down with a plane to a bendable thickness, about 2 mm.
It's fun to see the curls coming out of the plane. I guess you could use them for a little doll's hair.
Below is one side of lining is glued into place, the other waiting for gluing. Clothespins reinforced with rubber bands make great lining clamps.
The linings do a couple of things. They help make the ribs a bit stiffer, with out filling the whole width. They also create a wider edge than just the rib itself, giving a better gluing platform for the back, in this case, or the top, on the other side.