I made my planter box very similar to this last summer and a relative asked me to make her one so I said sure anyway I owed her big time for minding my kids as I went away for a weekend... I made hers a little different than the one I made myself and the main difference is that my planter box walls I used the wood horizontally and I made her planter walls vertically as you can see..
It came out awesome ....by the way I made a blog on my project here....
The planter box frame is made from 2×2
The planter platform is made from 2×6
The planter walls is made from 1×6