Small Scale Military Headquarters

Well I have never made a plane out of the green foam but I have used it to help my son with a school project when he was little. It is VERY soft. So soft that I was able to shape it with my fingers. It has no strength what so ever. As a mater of fact we painted it with an acrylic paint and the paint brush left marks in it.

I'm sure it would be doable, but how durable it would be is another thing.

Don't they use foam core construction in RC aircraft? Where they sandwich a foam core between balsa, or fiberglass.

you should be able to get different grades of that florists foam. The stuff that pickelhaube mentioned is one type. I've been to a florist and they gave me this other green stuff which was much more brittle.
When I was at uni we used this other foam which was light blue in colour specifically for model making. You couldn't "rub" it with your fingers, but it would dent if wacked against a table edge or something. Maybe you could contact some architectural model makers if they have any suppliers- or even offcuts

that foam if wet it disintegrates so it will be better to use other foam or other material as fiber glass