I am renovating an old door (pictures below), and I no longer want the letterbox, so have taken it off. Obviously I am left with a rectangular hole which goes straight through. I would like to fill this so when I sand and paint it later it will be invisible, as if there had never been a letterbox there. How do I do this? Any help greatly appreciated.
http://imagehost.darkernet.co.uk/i/door1.JPG
http://imagehost.darkernet.co.uk/i/door2.JPG
http://imagehost.darkernet.co.uk/i/door1.JPG
http://imagehost.darkernet.co.uk/i/door2.JPG