Then the men of Israel said to Gideon, Rule you over us, both you, and your son, and your son's son also; for you have saved us out of the hand of Midian.
In making this ephod, Gideon invaded the sacred precincts of that which belonged exclusively to the High Priest of Israel.  In  Exodus 28:4ff and 39:3ff, one may find the Divine directions for making the ephod.  It was a vestment to be worn exclusively by the High Priest.