Hiya. I do a lot of ships in this style etc. I would reccommend a couple of approaches which might work for you.
At that relatively small build size (around 9 inches) you can probably forego the spine if you want, although a piece of square section plastruct or similar would help you keep everything aligned correctly.
It looks like those designs would break down into modular components. You could build each section and then bond them all together. Whataver you do you should definately think about using the cross section idea. if you use a central beam you need to anticipate this on your digarmas before cutting your sections so the hole is always central.
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Kind of like that if you catch my drift.
Hope this helps.
Paul M.