Iv done sometihng close to this.
Iv made some small walls from various pieces of chicken wire and spray on foam. Iv used it to make better enclosures for snakes and lizards inside various aqauriums.
I use some stryrofoam and some spray on foam and carve it with an razor knife, spray on some acetone and then paint it all adn seal it.
Its good for stuff like that and Iv been debating making somethign larger for a backdrop to an indoor garden Im thinking about making. The problem being that what yuor talking about doesnt hold well in outdoor places.
the sun, the winter, the thaw the on and on and on.
Maybe you could paint it all up and seal it and put it out for summer and store it for winter and have it last longer?
Being new here I dont know any of he members or what they are into and how they have been around so with out trying to insult you, Id recomend taking a look at the hypertufa section of this forum. Im sure you can gain enough insite there to aid you in creating something that will last outdoors for years to come.
You could use the foam as the base and build up layers of the hyper on it and there are plenty of tips in that section on makign it look more natural and like real stone.
If your an regular user of this forum, and know all about the hyper, I apologize for filling you on what you already know, if not then glad I could help ya to find somethign that may work for you.