Boxes are pretty simple to make yourself (they're only six-sided after all) and when you make your own, you get to decide the dimensions, rather than having to make do with whatever you can find.  When I want to make a box, I just get out a sheet of thin cardboard (cereal/potato chip boxes usually), ruler, 90-degree angle square/guide, and a pen, draw it up, cut it out, fold it, and glue the edges together.