i got a similar method that also works
put the new fish into the betta box, u will start to see which is the fish that will want to challenge the new fish
depending on what type of betta box u have ,
mine has side sliding doors , so i open the door and the agressor will swim in then tadah, new fish goes into tank, agressor stuck in betta box for 2 days, after that once u release the agressor it will no longer be agressive