Lightweight joint compound is available at Menards (Ring-Ring) and Home Depot etc. Get a small container or two.
After you secure the chunks of Styrofoam with Great Stuff in a rough shape of what you want; Then HAND apply the LWJC over the Styrofoam. Do worry about the details just lob it on. Then using the "Di Vinci Method" spray paint while wet and toss on a lot of grass/earth mixture. I use a aerosol spray to get the grass mix on the entire thing.
Later you can spray adhesive and add even more grass/earth mix or spray paint more light earth tone colors or plunk down trees or big rocks. Knock your sox off! but not on the mountains!
I sometimes spray a bit of black on it and then a light sprays of silver or gold paint to give it a reflective quality.