Well, this is the Mystery Box from Call of Duty: Zombies. It works pretty well, but I had a few errors with it dispensing out both a strong and weak weapon in one go. I couldn't stop it from activating twice for some hits, so I balanced it out. Some of the main parts of this are:

1) Mystery Box is fully randomized
2) Point/currency system of kills, and you get 1 point for every zombie/mob you kill.
3) Detects when you're close to the box, and will only dispense an item if you're close to the box.

You can actually incorporate this box in a map, if you'd like... just give me credit for this. Sorry about the over complicated redstone. Also, if anyone knows how to fix the multiple items problem, please tell me! I'll give you credit for it in this post. :)

-Steven