Air lock designed for in-bottle fermenting, originally I designed it for fermenting cider in glass bottles but it should work with any bottles with a mouth diameter between 24 - 30mm (potentially others if you scale the model before printing)

To use simply fill with water from the top then insert tightly into neck of bottle (some silicon tape/rubber band/ teflon tape would all help in making a tighter seal but I haven't had any issues using the print as is)