Large Scale Scratchbuilt Planet Express Ship Material Questions

If I am going to be making a large scale scratchbuilt spaceship (1:24 in this case), the Planet Express Ship from Futurama to be exact, what should I start with? My problem is that I want the interior fully detailed so an inner network of supports will have to leave room for interior finishing, but the rough overall dimensions will be about 6 feet long and 2+feet in diameter. If I were to make all interior walls and floors sufficiently thick (I'm thinking 1/4 inch might be enough) and reinforce large unsupported areas with a rib system between an outer and inner wall, and finish it off with a fiberglass outer shell secured to that, would the model be rigid and sturdy enough? I will also be slicing it into three pull-apart levels, but I think the method I will be using, a slot and tab method with cross-pins to secure it, will not affect the rigidity as long as the construction I outlined above will work. I am trying to find if there is a lightweight but rigid plastic/styrene material that I could use for the inner support floors and walls, or if something like a high grade 1/4 inch plywood would just be best. Any thoughts on this would be greatly appreciated.

Thanks