I rarely fold boxes 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.
Can be made from square or rectangular paper. A box and matching lid can be made from same-sized sheets.