Well, it's kinda impossible to see what I've done here, but I'm making progress.

After seeing how much nuts and bolts cost on eBay, I decided to make a design that zip-ties together. At the moment this works, but it's flow rate is less than desirable (though probably good enough for as fast as I want it to flow) and the 3D printed fitting that I made to screw onto the bottle and connect to the drain hose leaks. It looks like it's just coming through the threads though so I might try some thread tape, but if I can't stop it from leaking then this is 100% unusable because I can't have it leaking water everywhere.

I tried checking the 1" pipe fittings at the hardware store, but they don't fit the roughly 1" pop bottle threads.

I made this 3D render that shows it not much better.

There's also a thing that goes inside the larger bottle and surrounds the smaller bottle to hold it in the center and upright, and a similar thing that goes inside the smaller bottle and around the siphon tube.