As posted above, styrofoam works great, Also you can put over it Textured Paint, from Lowes, Home Depot etc. Just brush it on, (Fine Sand version) and it adds a good finish to it. And then you can paint and weather it.
To join two pieces of styrofoam together, put Liquid Nails on one side, and also push in 2" or 3" wood screws. They will hold the foam together while the liquid nails sets.
Used the above methods many times in building 1/6th scale projects.
Look forward to pics.
Have fun!
-Rex