Defence in the Flight Deck
It seems unlikely that we will be allowed any form of weapon. Some years ago I purchased a large 'Maglite' which doubles up as a truncheon ala US police, I was thinking more of uncontrollable air rage than hi-jack, as we are supposed to be passive with hijack. It's inoffensive and you cannot be stopped from taking it on board and it's unlikely to kill anyone.
It is unlikely to be ever used like that - but if your prepared - it seldom happens.