Under the compass, i.e. within the square and hollow space of the altar. That the net may be even to the midst of the altar, or, and the net shall be at (the Hebrew ad being here used for el, as Hosea 14:2 Joel 2:12 Amos 4:6,8), the midst of the altar. And these words seem added to explain the word beneath, to show that as it was not to be at the top, so neither at the bottom of the altar, but in the midst of it.
Related Commentaries of Exodus 27:5
Exodus 27:5
5 And thou shalt put it under the compass of the altar beneath, that the net may be even to the midst of the altar.