How to get Shiny boxes from Siege Battles

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Since I'm in a rather small guild we don't have the manpower to regularly earn Shiny trophy boxes (or to dish out enough punishment to get far enough).
As it seems they become available from B- grade and up and the higher you're ranked the more boxes you get, but who is actually eligible to open them? Is it completely random or first come, first serve or do you have to do a certain amount/percentage of damage in order to become eligible?
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