I had been looking for pretty wooden planters for the front porch and couldn't find anything affordable when I came across Ana's plans for a planter box. So I just made it a square and made it a bit taller.
I used finishing nails and glue to put the whole thing together. If you make the sides 17 1/4" then the bottom is wide enough for three pieces side by side. I added four pieces the same as the strips that were made to the bottom so that they stuck out into the center of the planter. Then I laid the bottom pieces on top of those. I left them loose so they could be replaced if needed after a few summers of water and dirt. You could also put a liner in. The width is just enough to give space for water to drain out.
Out of 5 pickets I could have made the side strips a bit longer and put a strip across the bottom. But I had a couple other projects in mind for the scraps so I didn't.
With the left over scraps I made the small flower box for a table on my deck. I glued and nailed that together too. So for less than the price of one planter box I made two and the small flower box. If I hadn't cut one piece wrong the first time I could have made a second small flower box.