I built a square foot garden last year with cinder blocks. I was very careful to level them, so the back ones are just sitting on the ground, and the front ones are flush with the ground. I have a very hard clay soil. The turf barely grows. We get a lot of moss in the yard. I hoped that building a bed would allow me to avoid the obvious drainage problem, but it did not work out that way. My box filled with water and most of my flowers rotted. I want to try again this year. How should I do it, so that the bed will drain, and the plants will thrive?