This shows, as the mural heads back to the tunnel, the flexible foam mountain material purchased at Allied Model Trains, in Culver City, CA.  It is already the perfect color.  They also sell powdered greenery to sprinkle onto the tops of the rocky surface with spray-adhesive; although I opted to use the same colors of paint to dab on.  This piece I had just cut with strong scissors to fit around the block of wood around the tunnel (see next pic).