Shaping Internal Stems
I pre-shape the internal stems before installing them on the forms. I start with an over-sized piece of wood so I have material to hold on to while shaping. The wood can be any light-weight species. In this case I used pine that I ran through the thickness planer to bring it down to 1/2" thickness. Cedar would work as well. You could use a hardwood for strength, but this area is actually pretty strong even without an internal stem. Mainly the stem serves as a solid point to attach the strips while build.
Use the bevel-line on the end forms as your stem pattern. Cut out the end forms to the outer line, then cut off the stem pattern by cutting at the bevel line. The stems often come to a sharp, tapered point at the bottom and/or top. There is no reason to make the internal stem this pointy. Instead I mark of a little radius where the pattern starts to get narrow and sharp and then just cut it off short.
Trace the pattern on to your stock. Try to align the longest axis of the pattern with the grain of the wood you are using. Extend the ends of the pattern off the edge of the board in a smooth curve. Mark the inside edge of the form but don't cut it yet. Cut out the stem, following the outer line.
Back at your workbench, flip the stem over and trace the inner line on to the back side. This way you have the amount of bevel indicated on both sides. Mark a centerline down the middle of the edge of the form.
Use a block plane to create the bevel. Plane a flat bevel, adjusting the angle so it touches the centerline and bevel line approximately simultaneously. Work incrementally. Make a straight bevel then work it into a curve following the bevel line. Shape both sides
When you have finished shaping, use the stem pattern to restore the bevel line on one side. Cut off the stem on the band saw, following the bevel line. Clean up the sawed edge with a little sanding.