Adam Clarke on Exodus 27:4

Thou shalt make for it a grate - Calmet supposes this altar to have been a sort of box, covered with brass plates, on the top of which was a grating to supply the fire with air, and permit the ashes to fall through into the siroth or pan that was placed below. At the four corners of the grating were four rings and four chains, by which it was attached to the four horns; and at the sides were rings for the poles of shittim wood with which it was carried. Even on this there is a great variety of opinions.