The only way I can see these epic drops returning, is making it rare AND difficult.
I can only think of 1 mechanic in game that fits this: The Achievement Bosses.
First you have to make 1 spawn, then you have to be lucky to get an epic box.
Just make sure that 1 in 100 (or less 
) drops are epic.
This would make it an Asian grind-fest if you want to hunt these boxes. 
But we can hope, can’t we? 
