Build A Cardboard Box

Step 3 Cut to Size

Now, push the item towards the top or bottom of the cardboard so you can cut it down to size. Do not push the item to the exact edge! You will need to leave half the width of the item's side as extra cardboard that will be able to fold up onto the sides of the item.

And leaving half the width of the item as extra cardboard on the opposite side, cut the rest of the excess cardboard off completely, from side to side.