I've recently had a gas fire removed leaving me with a cavity in the chimney breast. I don't intend to use this as a functioning fire, just decorative. I bought an old cast iron fireplace and the fire place slides into the cavity nicely with the square flat bit surrounding it against the wall. My question is... Do I need to chisel out the plaster to fit the the whole thing flush with the wall or just use concrete adhesive leaving it proud and buy a made to measure mantel/surround to cover where it meets the wall?