The passage I was hunting in the Tao Te Ching. Still not about boxes.

Thirty spokes around the hub:
In their nothingness consists the carriage’s effectiveness.
One hollows the clay and shapes it into pots:
In its nothingness consists the pot’s effectiveness.
One cuts out doors and windows to make the chamber:
In their nothingness consists the chamber’s effectiveness.

Therefore: what exists serves for possession.
What does not exist serves for effectiveness.