So I searched around for something cheap, easy to cut and glue-able to make the exposed sections of the pressure hull.
I found those plastic glitter glue bottles on sale for a couple of bucks each at the clearance table of the art store. Decanted the stuff into jars and gave it to a friend who has little kids. I'm sure he just hates me right now...

Here's what I was going for.

The thing really does taper at the ends, and have a double-bubble section in the midships.

Added plastic strips for the ribs, which were external on the XXI.

We had a discussion over on FSM. Were the internal spaces, i.e. the ballast tank areas, dark?

Or light? There seems to be ample evidence for either, and after all it's not something that probably gets looked at much. But these sectional construction methods kind of show. There were three camps- black anti-fouling like the outer surface of the hull, red primer which the Germans certainly used a lot, or some kind of chromate. Black wouldn't do, since there would be no way to see the detail, I wasn't sold on chromate as an attractive solution, so...
Next, assembly.