Could be XX% drop chance per player that attends. For example, you give it 1% per player. If 100 people are in the battle, it is guaranteed that 1 random person will get it. If it's 200 people, 2 people will get it. If it's 125 people, 1 drop is guaranteed, 2nd one is 25%. Just like product drops work.
But, no matter how the drop works, the most important 2 things are these:
1- Nobody should get the feeling of "Ah, I'll never get it, those high levels that overperform will get it." The reward shouldn't be tied to the FF performance.
2- The drop needs to be actually relevant. No wooden, iron-clad or steel lined boxes. Either a bag like we get in Christmas, or something else that will not be sold to the mobile trader when obtained by an average player.