1)
What is the meaning of "N'vuv Luchos"?
1.
Rashi, Rashbam, Seforno and Targum Yonasan: It means that the Mizbe'ach was made of boards of acacia-wood and was hollow in the middle. 1
2)
If, as the commentaries explain, the Mizbe'ach had no cover, how did the sacrifice on it?
1.
Rashi, Rashbam and Targum Yonasan: Whenever they encamped, they filled it with earth. 1