I've never participated in one of these leaderboard events as I never do jobs or attack rivals. I received only one event container one time when I accidentally attacked someone while scrolling rivals list.
I also like to keep XP gain to a bare minimum.
Is there a way to participate in the leaderboard events while gaining minimal XP? Is the challenge in finding the containers (eg. low drop rate) or opening them to reveal the event item?
To draw a parallel with epic boss events, It only costs me 1 XP to trigger an epic boss. How many XP does it typically take to find a container?