Keep an item after dying

I attempted to make a little box that you can carry around and open with a right click, simple enough. My idea was, that if you held the box in your off hand when you died, you would keep the box after dying, along with its contents, but I can't figure out how to do it. I am quite sure you have to use the "entity dies" and "player respawns" global triggers, as well as some player_persistent global variables to store the items, but I still struggle with getting the procedure / procedures right. What could be a solution? Any and all help would be great.