I rarely fold boxes [update: no longer true], but recently I needed one and could not find any satisfying model online, so I designed this one. In contrast to most models I could find, this one is completely plain on the outside (no decoration at all), quite sturdy, and it uses paper effectively (only two layers on the sides). There are several similarly good variants of the internal lock which hold the corners together.
This model can be made from square or rectangular paper. It can also be folded with arbitrary proportions of height relative to width and depth. This allows it to be conveniently used together with decorated lids as the bottom part of a two-piece box. You can also make a two-piece box using two folds of this model. A box and matching lid can be made from same-sized sheets by making the bottom part’s walls a bit taller (which at the same time will make the other two dimensions slightly smaller and allow a perfect fit between the two parts).
The walls usually buckle a little on the inside, but the outer layer remains perfectly flat, so this box is best for uses where you care more about what it looks on the outside than on the inside. If you need a box with similar properties, but using less paper (at the cost of strength), have a look at Thin-Walled Box.