Photogate IR LED or laser
For lowest cost of implementation you want to try using the IR LED and IR Detectors. Lasers would add expense that you really do not need for this. 

Since the IR LEDs emit a wide spreading light pattern you have to devise a system to deal with that. This is typically done by pulsing the IR LEDs on one at a time and the repeat. During the time a specific IR LED is on you observe the output only from the IR Detector that is directly across from the IR LED. This gives you the selectivity that you need and eliminates the problem of multiple detectors picking up signals from the emitters. As long as you place the emitter/detector pairs close enough together you should be able to detect the ball sized object. 

The challenge of a such system is selecting the components and designing the circuitry such that the multiplex rate of the emitter/detector pairs is quite a bit faster in time that the object you are trying to detect. Along with that performance goal will the taking a close look at whatever IR detector you hope to use and select one that does not have too long of detection time.

You can even make such system work as a grid by placing emitter/detector pairs across the box at right angles to what you have now. That would come into play if you needed to know just what region within the mouth of the box that the ball entered.