Progress on the first baseboard continues (slowly!) I now have some diagonal bracing, which has made the board pretty rigid. I have also added a sandwich of scrap ply in the corner where the inner baseboard edge and the backscene will curve sharply round to the second baseboard.
The plywood sandwich is bottom right on the photo below. I shall cut this to its final shape when I know exactly where the curve should be. The scrap ply will not be in view when the board is finished.