Hollow with boards - Slabs, or planks, rather than boards. The word is that which is used for the stone tables of the law Exodus 24:12; Exodus 31:18, not that applied to the boards of the tabernacle Exodus 26:15.
The brazen altar was a hollow casing, formed of stout acacia planks covered with plates of bronze, seven feet six in length and width and four feet six in height. Jewish as well as Christian authorities have supposed that, when it was fixed for use, it was filled up with earth or rough stones. If we connect this suggestion with the old rule regarding the altar of earth and the altar of stone given in Exodus 20:24-25, the woodwork might in fact be regarded merely as the case of the altar on which the victims were actually burned. The shelf round the sides Exodus 27:5 was required as a stage for the priests to enable them to carry on their work conveniently on the top of the altar. Hence, it is said of Aaron that he came down from the altar Leviticus 9:22. According to rabbinical tradition, there was a slope of earth at the south side banked up for the priest to ascend to the stage (compare Exodus 20:26).

Hollow with boards - It seems to have been a kind of frame-work, and to have had nothing solid in the inside, and only covered with the grating at the top. This rendered it more light and portable.

Hollow with boards shalt thou make it,.... The frame of it being made of boards of shittim wood, there was nothing within side but the grate, which was put within the square, down into the middle of it, and so was light of carriage; though the Targum of Jonathan, and other Jewish writers, represent this hollow as filled up with dust and earth, to answer to the altar of earth Moses was before bid to make; but this seems quite contrary to the present direction: the hollowness of the altar may denote the emptiness of Christ when he became a sacrifice: he emptied himself, as it were, when he became incarnate, of all his greatness, glory, and riches, and became mean and poor for the sake of his people, that they through his poverty might be made rich, Philippians 2:7.
as it was showed thee in the mount, so shall they make it; or, "as he showed thee" (g), that is, God. Moses had a model of this altar showed him, and he was to be careful to instruct the workmen, and see to it, that they built it exactly according to the model.
(g) "fecit videre", Pagninus, Montanus; "ostendit Dominus", Junius & Tremellius, Piscator, Drusius; so Ainsworth.