Good luck with that...
Since the chests have come out, I've killed six battlerays. Received one chest from it and the contents were..
2 jump chip credits
1 log disc
I've gotten a design off of a lordakia in 2-2 and from a free box (one just randomly floating in space
) in 2-5 so I really don't think the level of difficulty in the alien has any bearing whatsoever on the contents of the chest or the probability of an alien dropping a chest.