Don't know if this helps, George, but in building a mountain tunnel for my layout I started with the extruded foam as a base and added the Great Stuff spray foam to get a more rounded shape and, rather than using quilting material, I applied WS plaster sheets over it to get the final shape I wanted and then painted and added various turfs and foliage. Also, you should know that the spray foam comes in two varieties - one that expands quite a bit and one that doesn't, so you have a choice.
