Here is the principle:
I cut the leftover plant carriers and placed them upside down in the bottom of each bin. We cut the PVC pipe to fit from the very bottom of the bin, under the plant carriers, to the top of the bin.
I added some rocks to hold down the bottom barrier and take up some room as we filled the bins with soil.
Final price tag for three Earth Boxes (soil and trellis sold separately) would be $150 three of my "Grow Boxes" were only $20. Mine do not have casters, but that would be easy enough to do on your own if you needed to. I also realized I should have poked more holes in the plant carriers and then put a layer of newspaper under the soil for better watering, oh well, too late for me, but if you make one be sure to add that step!