I use 18mm nuts as bungs.
Shape the nut to the curve of the pipe.
Sand off/grind the metal clean on the pipe.
Hose-clamp the nut to the pipe, and tack weld.
Remove hose-clamp, complete welds.
Test for leaks by adding water to the bung hole.
When there isn't any leaks, I drill out the hole in the pipe.
