I cut up the foam pieces per plan (Page 14-7). They were pretty straight forward as the fixture boards. When it was time to carve out the local depression, I was a bit confused on which side was up, but I found my answer on the second paragraph of page 1, "Note that the top of the spar is flat" - meaning Part D was on top.
Per Page 1, Figure 2, the plan says 'sand CS2 or CS3 flush with the jig', I went ahead and sanded CS2 AND CS3 flush with the jig. To achieve this, I stood CS2 and CS3 tightly together, then I clamped a straight board with its edge parallel to the top of part D and sanded both CS2 and CS3 at the same time - giving me a perfect leveled top edge.
It is obvious that CS3 should be 90 degrees to CS1 and parallel to CS2. I cut up a bunch of 2" x 13" boards to be used as small fences to keep CS3 in place. Using a couple of angles, I positioned the fence boards and used a hot glue gun to hold them in place. It worked quite well.
If you look close, you can see the packing tape I placed on the fixture to keep the foam from sticking onto the fixture during cure.
Then I microed the foam boards in place. Weights were used to hold them down during cure. Note that I also used duct tape to hold both CS2 & CS3 against its fences.