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Right, I've been studying a side render of Thunderbird 5, and found forth a way to make up the saucer section, just by designing this template based upon the render.
Circles you see are where holes for copper tube structure, lighting and electrical wiring will be drilled, while also allowing air to circulate through them, providing light weight to the model.
Covering it over though, I thought of possibly using sheets of Balsa or Styrene coated in Isopon O.38 Body Filler which I got from my local Halfords. Once applied and sanded smooth, I'll be able to have a nice smooth shell to work on, as well as cut out some access panels which can be removable via copper tubing. Whence Nurnied over, you'd never have known it was there. Either way, it'll be perfect enough to add panel detailing once it's smoothed over.