Don,
The limiting factor is that to be able to use the "stock" EarthBox Company's AWS, the filler tube in a DYI design can not be any longer than 15.5". That means if you use a container that is 18" deep, you would need to make a 2.5" "cofferdam" around the recessed 15.5" high tube. That is why the Rubbermaid 31 gallon model with its 16" depth is just perfect for the unmodified AWS.
The 31 gallon Rubbermaids are approx 150% the size of the Company's model, and I just considered them too small to accommodate 2 large Beeksteak tomato plants, which is my personal requirement.
The AWS systems are in stock on their website (received my first 12 two weeks ago, and have another 12 on the way now).
Ray