My bag of missing bits shower up yesterday so today I have my altimeter bay complete! I glued the internal coupler rings slightly too far in to allow the bulkheads to clamp the coupler properly, so removed the washers on the main side and sanded about 0.5mm off the sled to get everything to fit/clamp nicely.
If I was to build a second one I'd make sure the rings in the coupler are a hair less than ¼" from the ends, rather than the hair more that I built. I think the error came from placing the ruler at exactly ¼" making with a pencil at an angle (trying to get inside the tube) which was probably close to 7mm (9/32") as a result, and then following the instructions without thinking and gluing the ring just under the mark I made, so ended up about 1mm too far in on both sides.
Other than that one easily fixed issue, everything went together really well! I had to do a test fit in the rocket as well of course, however I hadn't actually started on it yet so everything is literally just friction fitted together. Sitting beside my Big Daddy for scale.