Once the box objects are separated from any background noise, they can be duplicated and arranged. Above is an example of an arrangement of several box groups. The higher boxes are obviously set behind the ones in the foreground so they don't appear to be floating in space. In addition, the size of the more distant boxes is slightly decreased according to basic principles of perspective. If you want to get all fancy, you could create perfect cast shadows from one group of boxes to another. I didn't feel such detail was necessary, for the shading of the boxes is convincing enough. If a perfect shadow cannot be created, it often looks too artificial and isn't worth the time anyway.
The boxes above represent about one third of the boxes seen in the final composition. I simply copied the group several times. Obviously the color difference and perspective angle is quite apparent here, making it obvious where the two boxes photos I took separate. But who cares. I had limited time, and these details are mostly lost in the final composition. Once again, perfection is not the goal here.