How to fit your own letterbox
Trick of the trade
If your front door directly faces the elements, it’s a good idea to put a slight slope on the cut you make through the door, so that the opening on the outside of the door is ever so slightly smaller than the one on the inside. This way, any water droplets that find their way behind the outside of the letterbox won’t run to the inside and through onto the floor.
If you do this, you will need to ensure that the inside cover still fits over the opening well enough to get post through, so don’t make it too small.