I have built the main forge chimney out of Blue Foam and in three pieces. Each piece was cut from some scrap foam. I have first carved the random stonework with a scalpel, then textured the surface by pressing a rough stone into the foam and finally sanded the surface with both sandpaper and a wire brush. The chimney top has a thin lip (a fourth piece of Blue Foam) although I am still undecided on how I will model the chimney top or chimney pot.
This final image shows the chimney against the back of the forge (but still not glued in place). You can also see how I have further textured the Green Foam timbers, nicking small chips out of the uprights and adding additional wood grain and weathering. This will be highlighted once the model is painted (and particularly when it is drybrushed).
As will all of these entries, a similar post will appear on The official Lead Adventure thread, go to this link for more details.