AAron,
I toyed with the idea of using a mini-float valve from U.S. Plastics, and adapting it inside the water reservoir to control water level. I even bought 3 to try out the design. Frankly, the EarthBox Automated Watering System has worked flawlessly for me for the past three years that I have lost all motivation to deviate from this ingenious design, and attempt to replicate this functionality with a float valve.
At about $13.00 per EarthTainer, the EarthBox AWS is a godsend, in my opinion!
They could double the price tomorrow - - and I would still buy it. It has saved me countless hours manually watering my 31 EarthTainers.
Raybo