I currently have an open chimney breast, there is no fire installed and it has simply been used as a storage space for years so far as I can tell. I want to block off the chimney breast, but rather than have a flat wall I wnant to leave the alcove so we can make use of the space. Effectively I want to block off the top of the space so nothing can fall down; there is currently a lot dust etc. Am I ok to simply stick a bit of wood over the base of the chimney to block it off? Do I need to worry about ventilation or anything like that if it is not used? Help and guidance much appreciated