Question: How do I cut off the top of the box ?
Option 0: Draw with a pencil a line parallel to the top of the box and 3 inches lower than the top of the box. Then, cut on  the line with a band saw.
Option 1: Draw with a pencil a line parallel to the top of the box and 3 inches lower than the top of the box. Then, cut on  the line with a belt sander.
Answer: 0