I'm looking at all options.
I prefer aluminum skeleton and aluminum sheet exterior.
Weld the skeleton, use sikaflex for the exterior lamination.
Now, even that is expensive.
I have noticed used 16ft box truck bodies in fair condition for $2,500.
They have aluminum posts (they call stakes, aluminum siding, steel I-Beam floor frames, thick oak floors.)
They have a construction that lends itself to alteration with fabrication. On the surface the size is too tall.