Find some wood the same thickness as the door. Cut a piece to just under the size of the hole you are seeking to plug. Glue firmly into place with 'liquid nails'/ 'no nails' or its ilk. Any wood glue should do. Ensure you wipe the excess glue from the front and back surfaces. Allow to dry. Using a large filling knife, wider than the width of the letterbox, apply filler, using the two edges of the letterbox hole for a guide for the filling knife, to get the filler flush with the door surface (Ideally, you should leave the filler a little proud of the door surface, then sand the excess back once dry.) Once dry, sand. Apply more filler. Sand. And so on. Until you acheive the finish you require. Apply a coat of primer over the filled area. Decorate the entire door for best results. Tip: Once you have applied primer to the parts of the door you have just filled,you'lll have a far better idea of whether your filling is good enough. Ideally, you should not be able to tell, that there was once a hole there. If this is not the case, fill and sand again.