Just trying to think what that would look like. I suppose cut back the gas tube stub more, so the whole of it fits inside the gas block/sight, then tap the hole at the rear of the gas block? Currently the gas tube stub sticks out about half an inch, just so I could easily twist it into position. I think the tube stub needs to stay in place as without it the gas would blow out the sides.
I'm thinking this leak is gas blowing along the underside of the tube stub.
I don't think I have a specific safety detent set screw but I'm sure I could find some suitable set screw lying about.
Maybe tap and screw in with loctite.