Boolean Difference when the shapes aren't intersecting?

Hi all,

I am trying to boolean difference two shapes, but I usually use this command when a shape is intersecting. Please see my image below for reference - is there a way that I can boolean if the cutting shape is fully inside the first?

Excuse my terrible wording!


Screenshot 2020-04-15 at 19.19.38|690x432

Hello - no, there must be some connecting surface - if the goal is to make a mold, split the box first along the parting line into the two parts that would make the mold.