Шмот 27:8 Комментарий: Rashi, Chizkuni, Sforno & Rashbam

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, “make it hollow with boards;” it was to be hollow underneath in order for it to be lighter when carrying it. These hollowed out boards were inside of the mesh on which blood would be sprinkled, occupying the lower half of the altar. When the Israelites were encamped they would fill the hollow space with earth and offer the sacrifices on top of that earth. When they would move again, the accumulated earth would fall between the boards enclosing that hollowed space and the mesh, which looked like a sieve. Each time the came to rest they would fill that hollow space up with earth again.

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