I am installing an Aga into an existing kitchen fireplace. The chimney is lined with a 5" flexible liner and I am currently building a false ceiling where the central portion will be cementboard. My question relates to the stove register plate. Firstly whether one is necessary given that the flu will pass through cement board and directly into the liner. Second, given that the Aga flu sits fairly tight against the back wall, could I buy a standard steel register plate and trim the back edge or does this defeat the object of there being a heat shield/border around the opening. Cheers 27 minutes ago