Rashi on Exodus 38:7/רש"י על שמות ל״ח:ז׳

1נבוב לחות HOLLOW WITH TABLETS — נבוב signifies hollow, similar to, (Jeremiah 52:21) “And the thickness thereof was four fingers: it was hollow (נבוב)‎”.

2נבוב לחת — The word נבוב being in the construct state and subordinate to לחות the meaning of the verse is: He made it (the altar) out of the hollow space formed by the tablets i. e. the planks of Shittim wood mentioned in v. 1 were oil all sides and the hollow space in the middle (thus the hollow space itself — which was filled with earth — formed the altar. Unlike the golden altar it had no roof; cf. Rashi on Exodus 27:8).