The first problem I see with this is the fact that you are using an HOB overflow box and not a "more traditional" drilled overflow. The problem with these guys is they can be very finicky when it comes to starting, stopping, and starting up again. IMO the best way to block up and overflow is by squeezing a coarse sponge into the overflow that is slightly large than the space it is going into. That will provide some resistance against an overly determined octopus. The problem with that is that it would restrict the flow going into the HOB overflow and you may have issues matching the flow rates of the drain and the return. This problem would also arise if the sponge ever got dirty or clogged, in which the box would slow or stop draining, and you'd flood the tank and drain the sump. If the octopus you have or want is too large to fit through the teeth of that overflow I would just cut out an acrylic sheet that fits the entire length of the tank. You'll have to cut wholes the exact size and place of the U-pipes in the overflow, and the return pipes, but that would keep from having flow issues. You'll also need to raise the overflow box up to acrylic lid or put a sponge between the lid and overflow box to keep the octopus from climbing up and over. I see in the pictures you have it almost as low as the thumb screw can be. Just my two cents. Take it with a grain of salt and wait for more people to chime in.
