My layout hill terrain has been built up with pink board, smoothed with rolled newspaper and covered in woodland scenics plaster clothe. I am going to use some woodland scenic rock molds but will need to 'freehand' some outcroppings around a small cave. Have looked at Crayola, don't laugh, air dry clay product. It can be painted, it resists cracking and can be molded prior to drying. Any thoughts?