Thanks guys.
It's closed-cell styrofoam type stuff I'm talking about, to model a couple of moderately large drystone constructions.
For one I was thinking of using thin sheet and applique'ing it onto a base made of a.n. other material (e.g. polyolefin foam like I often use), for the other (a largeish ring shape) I was going to use a 50mm sheet, cut the ring out en bloc and engrave.