Exodus 38:4-7

4They made a grating for the altar, a bronze network, to be under its ledge, halfway up the altar. 5They cast bronze rings to hold the poles for the four corners of the bronze grating. 6They made the poles of acacia wood and overlaid them with bronze.

7They inserted the poles into the rings so they would be on the sides of the altar for carrying it. They made it hollow, out of boards.