Adding an Interior Backdrop
Adding an interior backdrop on the inside of the box can really finish off the design. Create a cozy little environment for your subject and add one. Keep in mind that the backdrop is obscured to a degree by the figure in front, so don't use an image that you want to be able to see all of it.
Here are some examples of backdrops created for specific box themes:
For a South Beach Miami Bartender
For a Manhattan Resident
For a Graffiti Artist
You may want to use a photo you took yourself from a vacation, etc., or you can find awesome images on Google Images. Just make sure you set your search for high resolution images.