John Gill Commentary Exodus 27:5

"And thou shalt put it under the ledge round the altar beneath, that the net may reach halfway up the altar." — Exodus 27:5 (ASV)

And you shall put it under the compass of the altar beneath ,
&c.] That is, the grate was to be put within the square compass of the altar, in the hollow part of it, for the wood and sacrifice to be laid upon it:

that the net may be even to the midst of the altar ;
and as the altar was three cubits high, this net or grate was let down by chains to its rings a cubit and a half, and being of such a depth was capable of containing a great deal.