I would make individual boxes and make them at least 3'x3'. Six or eight inches high would be sufficient and fill with a good quality soil medium. I would add a soil less mix, bulk if you can find it since you will need a good amount. If you can't find this, I would make my own using compost, sphagnum and a little peat. You want it as organic as possible and well draining. Then in the fall, add 6" of mowed up leaves over the top. This is the type of soil I use in my 4'x10' boxes for my tomatoes and hot peppers and it's the most organic well-draining soil I've ever seen. I just keep adding leaves every fall.