I loled. It's a tornado adapter for a pair of Gatorade bottles. These were pretty popular when I was the age my kids are now, granted they were for 2L soda bottles.

For this particular project, finding a model that already had the threads in it.

More broadly, threads usually follow one of a handful of standards. It's just a matter of figuring out which standard is being used and then finding the size. I haven't found a great way of approaching that for bigger threads, beyond measure with a pair of calipers, but if you're working on nuts and bolts a metal fixture with various threads is the way to go IMO.

Sorry to burst your bubble but these have been a thing since at least the 90s

Oh, I know. I was a child in this era and wanted something similar for my kids.

I just wanted something that could attach to a more sturdy bottle as 2L soda bottles are... pretty flimsy when a kid is shaking them around to get a vortex to start.

This is true, but also not a huge deal for this design. Store it upright and water isn't contacting the print. While in use there's not much prolonged contact, or even contact area, between the water and the print.

These weren't absolutely leak free, but they didn't leak any more than the injection molded pices for 2L bottles.

I had one of these years ago, and they usually start to leak after a while regardless. It doesn't provide a good enough seal with disposable bottles, and swinging it back and forth seemed to wear it out over time.

Even ABS?

Eh. I've had good luck with PETG, which won't break down in contact with water. It helps to print three or four walls, and some simple post processing really seals the deal (ha! sorry for the word play).

That's exactly what this is based on.