Progress! The centerboard case fits... finally! Of course, cutting the slot was a fine lesson of measure twice (at least!), cut once. I would have sworn (actually, I did) I measured more than twice. Despite that effort I cut the slot too long (good thing the boat is wood and epoxy - I can fix the mistake and paint makes such things invisible) and too narrow (that was planned - better too narrow than too wide). Tools used in fitting the case to the slot: block plane, rasp, pull saw and scraped knuckles.
Here is the case fitted to the bottom, the gap in the bottom at the aft end of the case clearly visible:
After getting the fit close, I removed the case, took the bottom off the platform and trimmed the risers to allow access for installing the chine (later in the process). Once the risers were trimmed I screwed the bottom to the platform using sheet-rock screws with 6mm plywood 'washers' (those darker spots on the bottom). Then I 'fine-fit' the case. And, for fun, propped Frame #5 at the aft end of the case.
If you look closely, you can see marks (put there when the frame was built a couple of months ago) on the frame lining up with the edges of the bottom. Isn't it great when parts line up the way they are supposed to?
Okay, so maybe you can't see it in this photo. Believe me, the marks line up with with the bottom edges. This is both very satisfying and very encouraging!
Now, the case has not been installed as there are one or two details to address before taking the Very Big Step of actually screwing and gluing it into place.