The purpose of a mold box is to contain the liquid rubber (after it is poured over and around a model) until the liquid turns to a solid. A mold box does not have to be a complex structure; depending on the size and configuration of your model, often a coffee can, cake pan, plastic bucket or paper cup will suffice. Some mold makers will use cardboard or foam core to construct single-use mold boxes.
If you make molds of flat, two dimensional models on a regular basis, there are a number of advantages in constructing an adjustable mold box. The video below demonstrates how you can use acrylic or wood board to construct an adjustable, reusable mold box.