The last step is to cut a strip of your 2nd piece of poster board to fit inside the box as the backdrop to your photos. Be sure to cut it lengthwise. You want to cut the width about 1/2" smaller than the width of your box so that it will slide right in. My box was 14" so I cut mine 13 1/2" wide.
Slide it in from the top of the box making sure that it does not crease. You want your items to appear to be floating and a crease would show up in your shots. The board should be gently curved then taped in place at the top of the box. I bought a black poster board in case I wanted to change it out for a different background.
If you look at my finished box photo you will see the curved piece in the box at the back.
Here is the finished box sitting on my bar. I have my tripod in position and a bowl for the photo. All I need now is to set my lamp up to shine in from above and I should be ready to go.