I would suggest putting the washer on the inside of the keg, a silicon washer and a luck nut on the outside of the keg.
like this:
except without the o-ring inside the kettle. The extra o-ring doesn't do anything except make the connection more wobbly IMO.
(Obviously your arrangement will be different because you have different components, but the concept is the same.)
pic taken from
Bargain Fittings