Pete, (Avanti), I decided to try your method, not for the spot I need to rework but as an experiment. I took a regular Styrofoam sheet and cut a strip, and then blocks for quarry stone wall. Glue on to a thin piece of chip board, painted and weathered with acrylics. Let me know what you think'..
Thanks again for the great idea'.. Showing the process for those that may want to try Styrofoam for stone blocks.