Standard water bottle cap with a stem going through it. The stem is not attached to the cap and it is not water tight so that air can be released as the bottle fills and the cap can twist while the stem stays fixed. I designed this to be used with a gravity water filter that uses the larger Camelbak Crux or 5/16" ID tubing.

Print it upside down with supports so that the two pieces will not "fuse" to one another during printing.