Step 1: Assembling the Box
The metal pieces I used to assemble the boxes were corner braces and flat braces that I found in the deck building section of the hardware store. I used galvanized screws to hold them together so the screws won't rust. The plastic lumber has a long life so I expect the boxes to last for many years. They have already been out in sunlight and all weather in another yard for eight years with no obvious wear.
You can make the boxes one, two, or three boards high depending on what you are going to plant and how far down you want to bend while weeding.