According to the rulebook, contact can be made below the knee:
"Contact with a fielder is legal and interference shall not be called if the runner makes a legal slide directly to the base and in the baseline extended (see diagram)."
As a catcher, I was always taught to put my foot six inches above the plate on the foul line. That guarantees the runner can't get to half the plate at all without blocking the plate without the ball.
Then once I had the ball, by dropping my knee, I would cover the whole plate.