5th dnd – Should a lich phylactery be a box?

From the description of the lich on page 203 of the MM:

A phylactery is traditionally an amulet in the shape of a small box, but this can take the form of any object having an interior space in which are engraved in silver seals of name, binding, immortality and black magic.

A mirror probably does not count as having "an interior space in which arcane seals are engraved," but if you are the SM and you want the phylactery of the lich to be a mirror, you have the power to do so. Rules are a guideline; they can be folded or broken as they see fit.