We'll ditch the term inside/outside the box and instead dive into the principle of the "Closed World". A place where creative ideas exist.
The Closed World forces us to operate under constraints that increase our creative juices. This plays out in all walks of life: soldiers in the military, who are out in the field with no access to supplies. Business executives who have a set number of resources available, but have to deal with a competitive landscape. Even astronauts in space.
Once you understand this concept, you'll be halfway to thinking creatively, whenever you need it.