"In addition, there are no planting shelves, and no flat bottom to place the pots on. They tend to tip over."
At 3' deep you should be able to build a platform to set a container on. I use galvanized stock ponds which are about 3' deep and I've used cinder blocks, but my favorite are cat litter buckets. They are plastic, hold 35lbs of litter when new and have attached tops which snap back on. So I just let them fill up with water and snap the lid on. That keep them from floating. If I need it to be a bit taller or want to hide one with a colored lid, I put a flat rock or one of the flat cinder block rectangles on it. I have also used leftover PVC septic pipe and cut it off to the desired length and then used stones combined with some kind of container to anchor it down and keep it stable. The stones being both inside and out (in the container) act just like setting a post in concrete. Sine septic line pipe has drain holes on one side, the pipe also fills up with water, giving it more stability.
