5/16-inch (8mm) Barbed to Male Garden Hose Fitting

Two versions, one barbed, one stubby beaded. 

I highly recommend you print this model with the barb pointed down. This accomplishes three things:

1. You'll need supports regardless. They might as well be easy to remove. 
2. This orientation preserves the inside flat face so that the gasket can make a seal (instead of it getting all mucked up by support leftovers).
3. The threads will be as good as they get (as opposed to printing the object on its side).