I made this automated watering bucket for my flock and it works well. I fitted a float valve into this 25 Lt bucket that I cut open. I then connect the bucket to a hosepipe in my coop. It was easy and cheap to make. The photos speaks for itself.
It has been going for almost 2 months now with no hick-ups. I unplug the hose pipe every 2 weeks to wash the container out. The water stays remarkably clean.
When the container is filled with water it is too heavy for the birds to overturn. It is placed on a flat, level surface. I have never seen any of them sitting on the container. I think is is too slippery for them.
I bought the float from a building supply shop in their plumbing department. You should be able to get something similar, if not the same. This is a South African developed float valve used in small applications where a normal float valve would not fit.