A window 43' long and 22' in height has been installed in an old door frame. The door frame above the window was left intact. A piece of cement board (backer board) 43" wide 24" in height needs to be installed above the window where the rest of the door was. The framing is not flush with the walls around the window. The walls on each side of the window has 1" x 4" boards. I have covered them with backer boards to install tile. There is an area 13.5" wide by 46" in height on each side of the window that will not be flush with the area above the window. Any suggestions?