In the interim here is something I'm looking at. A collection of stills from Just Imagine
and the first episode of the Flash Gordon
The first batch shows us the original full-size mock-up and its interior. The mock-up lacks the riveting detail that shows up later when it's used for the Flash Gordon
serials. We can also see that the ship had quite a different and more sparse interior than that shown in Flash Gordon.
One thing the mock-up shows is that it looks proportionately shorter than the miniature used in the film and in the serials. This leads me to suspect the reference drawings from The Spaceship Handbook
are based mostly on the mock-up. That said it's odd that the access hatch on the drawings looks proportionately smaller than it appears on the mock-up.
The second batch of stills shows us the interior as it appeared in Flash Gordon.
It's certainly a lot more detailed and not as spare as originally. The detail also suggest bulkheads and supports the idea of an inner hull with mechanicals between the inner and outer shells.
In both instances, though, it looks like the interior looks bigger than would fit in the exterior mock-up. Hmm, very much like the TOS shuttlecraft thirty years later.