Seeking help with understanding compression fittings

Hi everyone... I am trying to build a gravity fed watering system for my ducks. I want to have a 5 gallon jug up top, and quarter inch tubing running down into the coop area. I have a float valve, and tubing, and brass compression fittings. Everything is a quarter inch, and everything is fitting together really nicely.

However, I have no idea what part I am supposed to get that connects the compression fitting to the upper jug. There has to be a part that goes inside the jug, has a washer of some kind, and pokes through the jug, so that the compression fitting can screw onto it. Otherwise there is nothing to hold that piece to the jug. Reference Image

I have never built anything like this and I am trying really hard. I've gone to three separate hardware stores to just kind of look at all the pieces, but it is so disorganized and all the tiny shelves just seem to contain random parts that don't match the label. There are acronyms I don't know and can't find any forums for this specific project. All the animal waterers that are online require a hose or electricity and I have neither of those, hence needing it to be gravity fed.

I am starting to get really really discouraged :( I don't even know what question I am supposed to be asking. I just want to connect the tubing into a 5 gallon jug without it leaking out, but right now I just have a compression fitting stuck into a hole in a jug and nothing keeping it in place and all the water leaks out the side. Apologies if this is the wrong section to post this.. Thank you in advance if you have any advice.