The trickiest part of a Christmas letterbox is making the curved top.
The easiest approach is to form the curve over some timber slats with thin 3 ply Craft plywood. Also known as Aircraft Plywood, Craft plywood is made of high grade Birch veneers and is available in very thin 3 ply sheets down to 0.4mm thick. We use 1.5mm thick craft plywood as it gives a nice balance between good flexibility and strength. The ply is available in half sheets of 1220mm x 1220mm. If you need full sheets 2400mm long you are limited to a minimum thickness of 1.5mm in Appearance Grade plywood.
Once the sides of the Christmas letterbox have been cut and assembled, four or five pine slats are fitted around the curve of the letterbox top as shown in the picture below. Once the glue has dried the thin plywood is carefully glued and bent over the letterbox top and clamped in position. It will be necessary to staple the ply as it is bent to hold it in place.
Leave overnight to dry and it is just a matter of filling and sanding the Christmas letterbox before undercoating and applying the final coat.