Why not just custom made a stainless steel letter box?

When my parents renovated the house many years ago, they included a built-in letter box. The problem is if you get those big letters aka like those annual report from listed companies, they would just fold it and stuck 20% of the tip into the box. Sometimes when there is a lot of letters, they also folded it half and also insert 20-30% of the tip. There are times when the postmen just fold it into half and stuff it between the grills of my gate.