I cut a four foot section of foam and clamped that to the keel batten. Then I took a Japanese-style pull saw (with a thin, flexible blade), and using the keel foam as a guide, cut the proper angle on the foam planks.
After all the planks were cut, I slipped in the keel foam so that it was flush with the top of the keel batten. The foam was about 2 inches high, and I cut back where the form frames interfered on the bottom using a Stanley Surform pocket plane. By the way, you can replace the Surform blade with a Micoplane blade for better/faster results. I finished up by screwing the foam keel in from the back side of the keel batten.