Scuba Inflator Elbow, works with DGX Bladder Flanges. I printed mine in PLA, and it leaks slowly (over the course of hours).
Disclaimer: for educational use only, has not been tested in water, use at own risk, etc.
I left the internal supports in the elbow (it flows plenty of air). Print the nut upside down, now supports needed.
Elbow is in FreeCad (file attached) and the nut is designed in Tinkercad. (I still haven't figured out threads in FreeCad).