Exodus 13 (KJV):
6Seven days thou shalt eat unleavened bread, and in the seventh day shall be a feast to the LORD.
7Unleavened bread shall be eaten seven days; and there shall no leavened bread be seen with thee, neither shall there be leaven seen with thee in all thy quarters.
8And thou shalt shew thy son in that day, saying, This is done because of that which the LORD did unto me when I came forth out of Egypt.
9And it shall be for a sign unto thee upon thine hand, and for a memorial between thine eyes, that the LORD's law may be in thy mouth: for with a strong hand hath the LORD brought thee out of Egypt.
10Thou shalt therefore keep this ordinance in his season from year to year.
11And it shall be when the LORD shall bring thee into the land of the Canaanites, as he sware unto thee and to thy fathers, and shall give it thee,
12That thou shalt set apart unto the LORD all that openeth the matrix, and every firstling that cometh of a beast which thou hast; the males shall be the LORD's.
13And every firstling of an ass thou shalt redeem with a lamb; and if thou wilt not redeem it, then thou shalt break his neck: and all the firstborn of man among thy children shalt thou redeem.
14And it shall be when thy son asketh thee in time to come, saying, What is this? that thou shalt say unto him, By strength of hand the LORD brought us out from Egypt, from the house of bondage:
15And it came to pass, when Pharaoh would hardly let us go, that the LORD slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the LORD all that openeth the matrix, being males; but all the firstborn of my children I redeem.
16And it shall be for a token upon thine hand, and for frontlets between thine eyes: for by strength of hand the LORD brought us forth out of Egypt.
So we have eating matzah for seven days in order for it to be "a sign unto thee upon thine hand, and for a memorial between thine eyes" in order for when your son asks you what it's all about you can explain. And then we have redemption of the firstborn which is also given as "a token upon thine hand, and for frontlets between thine eyes" for the same co-generational exposure and explanation.
C'mon now, how do these verses imply leather boxes instead of simply figurative phrases describing a mnemonic device?