Sforno on Exodus 27:8/ספורנו על שמות כ״ז:ח׳

1נבוב לוחות, similar to a box which has no rim around its base and no cover.

2כאשר הראה אותך בהר, that they were to fill the hollow inside with earth at the times they would be encamped. The fire [reserve of the wooden pile some of which was kept constantly burning. Ed.] which was to be kept going on the altar as described in Leviticus 6,6, would be based on the earth.