Hey guys speak of fittings, the best I have seen for your application, is the ones D-Tek has (not a plug...really)
There barbed, metal (coated - nickel I think), have a straight thread and an o-ring.
Real good ID, and safe on poly tops.
Hoping to start making a few thousand for them soon, but our CNC lathe is busy 24 / 7 making.... you guessed it... car parts
Looking for another one now that we have some more room.