Well another option is to make it so that the boxes fall out right where u killed the guys. Not sure if that would be any easier but thats another thing that may be possible.

I was actually just gonna post a topic on the same problem... I would really like it if this was put into the game as I was just playing and There were allot of boxes dropped in 11 and I only got 4 out of lets say any where from 10-20... I just bought membership for this very reason of getting boxes... now I cant get any of them... I hope this problem can be fixed.