I've seen individuals use those center pieces you have in there, as active vermipost tubes, throw in some screen, so worms don't migrate into the soil, and toss in bedding worms and food scrap. The vermipost leaches into the soil. If thats not what your already doing. I really like this design, especially the direct roof
watering, how do you plan on controlling the water flow? This could be great for urban settings, imagine
native flower cubes, where rainwater gardens have no soil, or not far enough away from foundations.