The container needs to be able to be air tight under a vacuum. It needs to be elavated somewhere above your sump/fuge/tank you want to Top off in. Wherever the water fluctuates. I used a water jug from a dog waterer. A rubber squeeze ball from a turkey baster, a male connetor(1/2in) a lockring to connect it to the jug. A barbed fitting and some 3/8 poly tube. Silicone dto stop any leakes. Just run the poly tube to the exact level you want the water maintained at. When the water level drops, it will suck air in and allow water to flow. When it rises above the tube, it will stop feeding water. A milkjug itself WONT work, because its too thin.
Rely though the driper method is pretty reliable, but I'm now using a float valve connected to my RO and t"d for kalkwasser.