You should do it so that every 4 boxes has the following:
4 "hit" slots
1 "base" slot
1 "premium base" slot
That would mean for every 16-box case, there would be 24 slots. You would be guaranteed a 1 in 24 shot at a case hit. The base and premium base slots would have added value.
This is how I would do it if I were to run a break.