One of those cute little things you put in bottle of vinegar/soy sauce/cooking oil/etc to allow you to pour evenly without the "gluging and spilling".

It has a tapered bottle stopper with 2 channels through it's centre One to let fluid out and the other to let air in.

I suspect that the ridges wont seal very well (partly because ABS is quite a hard plastic and partly because I have made then quite thick. However it would not take much to put a rubber washer on it to give it a good seal and hold it in place.

This design is untested.

The channels may be a little thin for some of the more viscous fluid (heavier cooking oils etc)