Another option that people can keep in mind is to use a washer outside the o-ring to keep it in place. Blichmann does this with their kettle fittings. The things are SOLID and the nuts cannot even be tightened any more even if you wanted to. There's no 'play' because the washer is in contact between the kettle wall and the device/nut on the outside.
For example, the weldless Brewmometers on my Blichmann pots came like this:
This gives you a super-solid seal that doesn't move if you push/pull on the Brewmometer. I really like this idea. I believe that Blichmann's trying to patent it too.
The trick is finding a washer that's a bit smaller in thickness than the o-ring.