Plain mitred corners are inherently weak, due to the propensity of end grain to end grain in the joint. For a box like this, I feel it is crucial to add some reinforcement.
Now you may ask, why then did I choose to plain mitre the corners? Wouldn’t I have been better using box joints, or dovetails, or adding a spline to the mitre? My answer is simply, I wanted a mitred box that was relatively easy for anyone to copy, and which had subtle decorative features.
My methods for reinforcement were to insert veneer keys, and through dowels. Either method being adequate, but both being a nice pleasing combination, in my view.
Thin strips of oak were prepared using the band saw, and then planed to a slip fit in a saw kerf. Then saw kerfs were cut across each mitred corner of the box, and the strips glued in. Remembering to run the long grain perpendicular to the joint line.
Next I drilled holes for 2mm dowels (hardwood cocktail sticks)...
...cut the dowels over size, applied glue and knocked them home.
The box was then left for a day, to allow a full cure to occur.
-- Design, Build, Inspire. http://www.WOmadeOD.co.uk