I have decided to build the centerboard case outside of the boat for several reasons. Primarily I wanted to be able to put the case on the drill press to ensure the centerboard pivot holes are perpendicular and perfectly in line. In addition, this also lets me glue on the flat, and I can use clamps vs. drilling screw holes and having to fill those in afterward.
I took quite a bit of time to ensure all parts were aligned, then made marks so I could replace things in the right place. I applied unthickened epoxy to the bare wood of the case ends to make sure they were saturated, and then followed that up with thickened epoxy on both the end pieces and the case sides.
Once the glue was applied, everything was very slick, but I aligned marks and then fired in a couple pins from my pin nailer, which kept the pieces from sliding around when I applied the clamps. That worked well.
Both these plywood pieces had a bit of a twist in them, so you can see I have the far right corner clamped down to the heavy table, and the leftmost corner pushed down with a go-bar against the ceiling to take out the twist.
Gentle clamping pressure was applied to avoid starving the joints, and the inner case corners were smoothed over with a long dowel. In this photo you can see the end of a couple long spacer sticks lying the long way in the case to keep the sides the right distance part and make sure there is no overall bowing of the part.
I also set up the rudder and centerboard on the drill press and drilled the pivot holes out oversized.
And filled them with thickened epoxy. After that's dry, I'll drill the holes out to accept bronze bushings.
To keep the installed centerboard in the middle of the case, I visited the local hardware store looking for some large nylon washers, but didn't find anything appropriate. So I looked around the shop and found this chunk of plastic I picked up at a salvage yard some years ago. I've already used a corner of this for something, can't remember what. I traced a circle and cut it out with the bandsaw, then did my best to resaw it the thin way to yield two blanks.
I faced off the plastic until it was smooth and about the right thickness (just over 1/8th inch), and trimmed the outside edge to make it round.
Then drilled the center hole.
Here's the plastic stock, rough blank, and finished part.
And here's a picture showing the overall size of the spacers on the centerboard. They are about 3 inches across, which seems a good size to me.