This was designed in Tinkercad to replace the bottle cap of my wifes bottle. One of our dogs get the cap and chewed it all up. Rather than buy a new one, through trial and error with the threads I got something that works. I have attached 2 STLs. One is just the threads with a basic top to show the space needed around the threads and one with a basic shape as the cap. Feel free to use this or improve it. With the tools I used, I was very limited in creating the threads so they are not perfect and they were designed on a trial by error process. I needed one, didnt find any designs anywhere and didnt want to buy one so I designed one.

If you were to print the one with the ball, you just need to take the rubber gasket out of the existing cap you have and pop it in and it works great. No leaks that I have experienced. If you don't have a gasket, the OD is 40mm and inside hole diameter is 25mm. its about 2mm thick, give or take if you need to buy a new one.

Use this design however you want. If you just want a custom cap or a replacement for yourself or think you can use it to make money, go ahead.

For printing, I would suggest using the best printing options your printer can handle, no supports. I used 80% infill myself because I wanted the durability (probably work with lower) and used food grade PLA (no additives). I say the best available because its a water bottle. You want to limit and potential places for gross stuff to grow. I have read that PETG is also good for printing food grade stuff. I have not tried adding a food grade resin coating so it you do, I have no idea if the threads would still work. Its not like there is a very tight tolerance or anything so it might if you are super paranoid.

This is something in contact with food though so make it at your own risk. I make no claims as to this being "safe" from microplastics or anything of the sort. All I am saying is, I use it.