The box I cobbled together from old pallets, dirt is a mix of potting soil and local dirt spiced with chicken poo.
For the more observant viewers, you may notice that the closest 'V' is slimmer than the upper square -- the pallets were a little short. I plan to plant smaller vegetables in those areas.
Even with the square foot method you can get some extra efficiency. Plant some radishes and carrots together. Plant some sunflowers and peas/beans together. A lettuce or two with a tomato.