The foam was textured by rolling a rough stone over the surface to dent it up a bit (a ball of heavy aluminum foil would also work). I filled gaps with lightweight wall spackling (from DollarTree).
The painting is actually pretty simple, too. I used black Gesso as a base coat (this was an experiment, I usually use black craft paint but the gesso does seem to cover, grip, and seal better in one coat). FWIW, spray paint propellant eats foam, so that generally isn’t an option.
Then I used a sponge to heavily blot on “barnwood” (a grey brown), and then a “stone grey” on top. Putting the brown as an undertone varies the color a bit more and makes the stone a little warmer in tone. I hit the edges with a brush with stone grey to highlight the edges.