1)
What is "Levenah"?
1.
Rashi: It is a brick. Some are big.
2.
Radak citing Targum Yonasan: It is a rock.
2)
Why did Hashem tell him to draw on a Levenah, and not on a tablet?
1.
Radak: Bricks are used to build cities, or rocks in place of bricks. It is easier to carve into bricks than into rocks.
2.
Malbim: The existence of Yerushalayim is only via Hashem's Hashgachah and desire. It itself is the formless brick. Put it in front of you; this hints that Hashem put [Yerushalayim] in front of Himself, to put the eyes of His Hashgachah on it. Carving Yerushalayim on it represents building it on its rubble.
