Box in Baby Sign Language is very similar to blocks, just a little larger. You take your hands open about 20 inches apart, and outline the sides of a box, first the left and right, then the front and back, then the top and bottom.
An alternative ASL sign for box is to reverse the order, first holding your flat open palms facing your body, one farther than the other to signify the front and back of the box, then proceeding to sign the left and right sides of the box, around torso width apart and parallel to each other.
Boxes seem to hold a special allure for children. As well as teaching your child the basic box sign, you can further challenge them to open and close the box. Or you can ask them to put something in the box or take something out of the box.
We have found that boxes tend to be the best part of any gift. If your baby shows a propensity to love boxes, teach the sign as soon as she shows interest.