Ours just fits snuggly over the top of our type of water-er, but with yours I wound suggest tying a string to the handle and a string onto a dowel. Then feed the dowel through the top (neck) of the large 2/3 liter soda bottle. Then tighten the string down with one of those knots that you use on tent stakes. (I would make a poor boyscout, sorry-A slip type knot) The small dowel should not hang over the cap area too much or they will roost on it. or maybe you could figure out how to just screw the cap on over the string.
I use soda bottles for funnels all the time. Trash/treasure and all that...besides any money is too much money lately. I don't buy it unless I have to. The plastic is too slick to grab onto and they are easily replaceable.
The pan we have is a hard plastic it is wider at the rim than at the base, so when it is turned over the water-er fits perfectly but has a stable base to keep it from getting nudged around. It is a bit higher than one cement block. I have Orpingtons, they are rather tall, I suppose it all depends on what height your girls are.
Our container was from a protein supplement for sheep. They scooped it up with their bottom teeth. Let me look on line and see if I can find it. Not that I would suggest going out and buying sheep food to store your water-er on.
Maybe you can put the base on a piece of cardboard, draw a template, cut it out and take your circle to the hardware store or Feed store and with the height of your girls in mind measure a bunch of hard plastic tubs flipped upside down.
I hope some of that makes sense. I am having trouble communicating lately.