^Yes, that makes sense about seeing the inside of the nacelles. Good catch.
IIRC, somewhere there was a series of a lot of B&W pictures taken of the model just before filming it in TMP. I have looked and have been unable to find them again. I imagine you are already familiar with those pictures, but just in case maybe there might be a good picture? Though as I recall, most of those were up close to capture specific details -- so the opposite of what we would need. What would be ideal is a good picture straight down the neck before the saucer was attached. Assuming such a stage existed.
I think someone will just have to track down the actual model with a set of calipers.