The Headcover Gate
Why this drill
The over-the-top slice swing crosses the ball from outside. A headcover placed a clubhead outside the ball makes that path impossible to take silently — hit the ball, miss the cover, and the club must be arriving from the inside.
Setup
A 7-iron, fifteen balls, and a headcover (or an empty range basket turned on its side) placed just outside the ball, about a clubhead's width away, angled slightly behind it.
The drill
Hit ten easy shots missing the headcover entirely. A clipped cover means the club approached from outside the line — reset it and slow down; speed hides the fault this drill exists to expose. When you can miss it five times running, hit five at normal speed. The ball's start line will drift right of your old shots at first (for a right-hander); that is the new inside path showing itself, and the face will catch up.
One thought to take with you
Swing out to first base — the ball is on the way, not the target.