Finally made some progress. Trimming the top and bottom edges is easy. Doing the front and back edges requires the (PassingInterest designed) router jig.
To prepare to trim the baffle side edges, I needed to cover all of the holes so the router had something to sit on. I used a piece of the hardboard cut to fit and brad nailed it down.
I wasn't as diligent as I have been in the past about setting the depth of the bit so that the curved edges end up "just" proud of the MDF face. I knew I would have to sand anyway, but that hardboard sands about as well as stone does and the result is that the MDF will wear down before the hardboard.
After "cupping" some areas of the MDF near where the hardboard meets, I got a bit impatient and made even more of a mess with trying things like using my brand new Hitachi hand planet for the first time. Bad move! The baffles will need some filling and sanding.
The good news is that the trimming on the back side and the top and bottom went great and I got to try out my new Bosch Colt router, which I like a lot. After some sanding, all is well, except for the baffle.
Moving on, I trimmed the top and bottom edges of the other box (the one that I used screws and furring strips on for the first layer on both sides) so that I could get that cab into the jig and get the rest of it's layers going.