When you insert it, people are sideways in it.

Depending on which one:

If you can look at your feet separately, it's two people. There is a black box under the box, so hide a person in advance. When the person that the audience can see becomes a box, the person in the black box sticks out his feet. The person who just turned in curled up. Then insert a knife ~ cut it and split it in half.

If the audience can't see the simplicity of the feet, the performer just needs to curl up and be ok ~

Magic is magic after all, and the general principle is props.