I mapped (hand sketched) out the final contour lines on the top surface, then scraped the inner surface to get to the final thicknesses. The most effective tool for this was (again) the round-bottom plane. Taking small bites with the plane, then sanding and sneaking up on the final dimensions.

As a proof-of-concept project, I'm reasonably happy with the way this turned out. Obviously, I did not take this all the way to the final finished surface. There was no need to go that far. The mock-up was a good idea. I found a few issues to stay away from on the next build. I'm going to go ahead and order some sitka spruce and get started on the drawing package for the real deal.