Rashi on Exodus 27:8/רש"י על שמות כ״ז:ח׳

1נבוב 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.