For those who may be tempted to build their own raised bed I thought I would post this in the hopes it would encourage you to give it a go. The lumber can be cut to size at any Home Depot or Lowes store. I just bought two 2"x10" eight feet long and had them cut in half.
On the inside corners I screwed two right-angle brackets (one at the top and one at the bottom) using 1 - 5/8" deck screws. You will need eight angle-brackets and sixteen deck screws.
I leveled an area slightly larger than a four foot square in my back yard and placed the wood frame on the ground. They say to not use treated wood because it is may contain arsenic and you don't want your plants to absorb any of that. I then placed several layers of newspaper inside the frame on top of the sod to kill it and to prevent weeds from coming up through the "soil."
As I mentioned earlier, rather than try to amend the awful soil we have in our back yard (mostly clay and rock) I used Mel Bartholomew's recommendation in his book titled
Square Foot Gardening. In his book Mel suggests using equal parts of peat moss, vermiculite and compost. Actually, I am an avid organic gardener and believe you can never have enough organic matter in the soil, so I use two parts compost, one part peat moss and one part vermiculite. I was fortunate to have a vermiculite manufacturer about 40 miles away in Woodruff, SC, so I bought it there in 2 cubic foot bags ($12 each). If you Google "vermiculite" you should find local sources that will sell in larger quantities than what is offered in your local gardening center (way too expensive). If you don't make or have access to compost, you can use mushroom compost or composted cow manure from the garden center. I suggest you purchase a section of what's called hardware cloth (heavy-duty screen) to sift the peat moss and compost. I purchased 1/2" square hardware cloth at a hardware store big enough to sit on top of my wheel barrow. Peat moss and compost usually have clumps and other debris (I've seen large clods of clay and big chunks of wood) and you don't want that in your "soil." I use a small shovel and turn/mix the ingredients inside my wheel barrel. Just before mixing I add about a cup full of organic fertilizer, a handful of dried blood meal (good organic nitrogen source) and a small handful of pelletized lime (all can be purchased at a garden center).
Fill the 4x4 frame full with this "soil." After each dump of "soil" from my wheel barrow I leveled it out and then wet it down with a hose. The "soil" is fairly light and fluffy and over time it will settle down so you may need to add more as time goes on.
After your plants are up you should mulch the entire surface of the 4x4 with organic material such as shredded leaves or grass clippings - about 3" deep. This will keep the soil cool during the hot summer months and it will help conserve moisture. In the fall, after the growing season has ended, you can take a spade and fork the mulch into the "soil" so it will add organic material. Before turning the mulch under sprinkle the surface with organic fertilizer and dried blood meal. For the first year it's a good idea to introduce some good soil from the flower bed or other area to the 4x4 as this will introduce needed bacteria that will help break down the mulch over the winter and early spring (sift it through the hardware cloth). This inoculation needs to happen only once as the bacteria are self-sustaining as long as there is organic material available.
Happy gardening...