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An octopus can get through a hole as large as its beak, and its beak is quite small. Bimacs are not as escape prone as some other species but they do go exploring from time to time.
I'm not familiar with that overflow, but I have a reef ready Oceanic Systems tank with an overflow that has holes around the top.
I've saved the mesh bags from fruits and vegetables (the harder mesh, not the softer one), washed and rinsed it carefully, and cut it into strips. I've woven these strips through the holes. It works! Other octo owners have come up with their own solutions or used mesh from different sources. It can't be too fine or it will clog up too quickly - as it is, it must be cleaned (with toothbrush) or replaced from time to time.