stitch and glue over a steel tube framework

It is not as bad as I thought it would be.
http://www.frpshop.com/styrofoam-c-39_40.html
The 60 PSI is $28.64 for a 2" 2'x8'
The regular square edge is $17.76
The problem is a lack of thickness options. I think your initial suggestion of 1.5" is right on to meet the thin side criteria.
So once the sandwich is built, it is much more stable with regards to expansion problems?
My intent is to finish it bright, at least initially. If I bollux that up I can always paint it with a pigmented linear poly.
My reference to "building it flat" was for the flat fixed lower sides. I get how to shape the foam, cut it in slices, fit it to formers, etc.

 

You don't have to cut it into slices, it'll bend easy over simple, easy curves. You only need to slice it if you have compound curves, like that seen on boats.

There isn't an expansion issue with sandwich construction, though what you screw or bolt these panels to, will need to address some expansion differences, which can be handled with a bedding compound.

 

I can get a 12" radius with 2"?

 

Thanks again PAR. I found 8x4 x3MM meranti bending ply that will easily do my 12" radius. I would use two laminations outside, to match 6MM ply on the flatter sections.
I had hoped to go with 4MM Okoume for weight savings, but finding a bending ply that matches up with that has not been easy.
This meranti is ~$20 per sheet.