What did Ezekiel set up as a wall between himself and the city drawn on the brick?

Answer: An iron plate

God told Ezekiel to take an iron plate (KJV: iron pan) and set it up as an iron wall between himself and the besieged city.

“Then take an iron plate and set it up as an iron wall between yourself and the city. Turn your face toward it so that it is under siege, and besiege it. This will be a sign to the house of Israel.”— Ezekiel 4:3 (BSB)