I am in the process of patching my 1912 plaster bathroom walls in preparation for applying Master of Plaster coating. One corner has a chimney which is no longer in use and is covered over above. There is about a 6 in stovepipe hole with the usual metal hole cover. I would like to fill this in and cover like the rest of the walls. I know I can use a piece of sheetrock, mesh, etc, but am wondering how can I put something in there to stabilize the sheetrock--or is there something better to use? I have a mesh patch which is somewhat reinforced to put over it, but I don't think it is solid enough by itself. Of course the surround is plaster over brick. Suggestions?