On a hike I came up with this little idea for a tool:
It's a set of curve transfer calipers. The rounded ends rest on the board and gauge it's thickness at any given point. The other end has little felt tip pens installed in threaded holes that will draw the shape to scale on a piece of paper. I only need half and then mirror it for the full frame shape. When that shapes are traced and cut out of marine ply, presto, perfectly shaped renditions.
The idea is to slide the tool perpendicular to the keel at 8" intervals (one for each frame). The elastic band keep constant pressure on the contact points.
I haven't actually tried it yet, but if I do I'll let you know how well...or how lousy they worked. If anybody out there has a better idea (or knows of an existing tool) feel free to leave a comment.
Now I'm sure you just can't wait to try these yourself. Feel free to download the full size plans here. and make your own set.