Step 3: Extrapolating from the Basic Shapes
A couple examples are below. I started by drawing a vertical grid, once again. This spacing was to make things wide enough for my laptop (going from center to right edge) with additional guides for later use. For each vertical slice along the spine, I transferred points to this grid. Transferring points is easy: I copied parts of the shape, dragged them directly over the new grid, and marked points.
The first example below is the front-most piece. I took the left-most line from the spines and measured out the middle spine on the left side and then the edge spine on the right. Then, I played with the bezier handles until I had a curve I liked and that could be copied across the rest of the shapes. In this case, I was simply drawing out the handles to be two guides inwards, each.
The second image below is the second cross-section; I just repeated the process for all the measurements on the second spine cross section.
After a bit of work, I had all nine cross-sections mapped out. I now decided that I wanted two more spines, to go in between the center and the edge spines. To do this, I found the height of each cross section between these spines and transferred them to a new spine (side) shape. You can see it in the third image.