3/4 inch concrete can be very strong especially if reinforced and high grade + fiber embedded + micro silica ect. ect.
One of the good things about concrete is there is a lot of documentation and info. Personally I use repair concrete like CR-60 (with some personal additives)  :-* very strong stuff

Well thats a good idea but when I think of concrete, I think of pretty big chucks of rocks and sand. But I have used that light stuff they use on floors in appartments. Its more of an epoxy than a concrete.
fiberglass and I go way back to the Car audio and boat building days. I bought a ton of Matt for my Imperial I never used and could use it inside and add a gel coat so make it glass like.
But thats too much effort and expense.

Placement test is a good idea and if I was to use concrete It would be an imperial sub but not a SO or a WO 32.

Man do I have lego! but not that much.

All ideas actually have merit, because even the wierd ones spark ideas.

I am going to a specialty wood working store to check some stuff out tommorrow in the big city.
( lee valley tools)

Can't start building until the Xmas bills are taken care of, so like I said I have time to really use the imagination.

Ferro cement is used in boat building and is very strong.One could build a collapsible inside form,cover it with a few layers of diamond lath and cover with a strong mpa motar.
It would only need to be 0ne or two inches thick so it would still be some what portable.
                         Brian