I wanted some larger flower boxes for herbs and some vegetables, so I decided to build some out of leftover pieces of wood from a garden fence.
I didn't have much choice for the dimensions, as I had to use what I had. Luckily, I had some nice 60cm long pieces and some 45cm long pieces. So the box became 60 by 45cm (+/- 23.5 by 17.5 inches). The wood itself was 9cm by 4cm (3.5 by 1.6inches).
Instead of screwing everything together, I decided to build the box in the same way as a logcabin. To do so, I had to cut notches in the wood with a depth of exactly ¼ of the width of the wood and the same width as the thickness of the pieces. In other words: 2.25 cm by 4 cm. When all notches were made (4 in each piece of wood except the bottom and top pieces that have only notches on one side), everything was glued together.
The bottom is made from a piece of 18mm multiplex and is attached with screws.