The best fix would be to separate the cases and have it welded but it might not be worth it. The only reason I can see why the JB weld did not work must be because the area wasn't clean enough for a good grip. Usually, it holds up pretty good. You might want to try it again with the bike turned upside down for the time it dries. Do some sanding before and clean with a solvent before applying the glue. I have seen some drain plugs leak because the thread was stripped. Is this why some people recommend you to use an expanding rubber plug? If the thread is stripped, you would be better having someone install a heli-coil insert.