Then measure the number of boxes done. Of course you have to be very alert for quality of boxes, and for selection of boxes. A savvy employee will only choose

Then measure the number of boxes done. Of course you have to be very alert for quality of boxes, and for selection of boxes. A savvy employee will only choose the easiest tasks as all are ranked equally. Exactly the same with ticket based jobs. Any measure you have can and will be gamed.