Two steps forward, one step back.
I re-cut the dados for the tongues — the joint that holds the sides together. I measured everything and the dados were about .035″ too shallow overall. I test fit the sides together and it all closed up nicely.
Then I cut a piece of 1/2″ ply for the back. Somehow I cut it out of square. I’m not completely used to the sliding table, and the miter gauge was slightly off — and slightly loose. I made a series of test cuts until I was sure it was dialed in properly, although I’m not 100% confident I have the table itself adjusted exactly right. It seems like it’s off about .030″ over the entire travel (about 6′), which seems pretty close — but it’s not 100%. I’m scratching my head about the best way to get that dialed in properly.
Anyway, I’ll have pick up another sheet of 1/2″ ply tomorrow and re-make the back. Then I can start fitting in the dividers and cutting more dados for the other shelves and cubbies. I think this will start coming together more quickly now. At least, that’s my theory.