שמות כז:ח פירוש: רש״י, ספורנו, רשב״ם ורבנו בחיי

נבוב לחת HOLLOW WITH TABLETS — Render it as the Targum does, חליל לוחין, hollow as regards boards (לוחין is what is termed an accusative of respect): boards of shittim-wood on all sides (cf. v. 1) and a hollow space in the middle; it shall not be in its entirety a single block of wood, the thickness of which in two directions should be five cubits by five cubits — like the trunk of a tree, which has been hollowed out.

נבוב לוחות, similar to a box which has no rim around its base and no cover.

נבוב לוחות, hollow. We find a similar expression in Job 11,12 איש נבוב לוחות, “a hollow man, etc.” Whenever the Israelites made camp they would fill the inside of this altar with earth. Only after that would they use it to offer their sacrifices.