I decided to try Vaclav's technique for filling in large areas where where the full-length shear strips meet the strips laid parallel to the keel. I let the ends fall short:
Then I used a full-length 1/2" strip to mark the cutout. I had little faith in my ability to follow the line with a saw, so I stapled it on & used it for a cutting guide:
This was harder than I thought. This technique requires a lot of skill with the saw. My pullsaw was too aggressive, so I resorted to multiple passes with a utility knife. This unfortunately gave a sort of V-shaped cut.
Planing the strip was more difficult than I thought it would be. At one point, the tapered strip would fit in like a wedge, but pull a neighboring section too far apart, etc,
The strip in place. It was not exactly a "Voila!" moment. Some gaps that will need to be filled. After all, I think It might have been better to fit each end. I dont think I'll be trying this on the deck.