Hatchlings, juveniles and dwarfs have been known to crawl into overflow boxes where we have not seen issues from adults. I highly recommend putting a bulkhead screen (link is for reference only. These come in both threaded and slip for common US bulkheads). The screens will still allow hatchings to visit the sump but will prevent most others from exiting this way. The weir DOES need to be covered. Placing a rough sponge where the animal would put its arms to investigate helps but can be a concern with the flow (and needs regular cleaning). A bright light over the weir can also help (especially with nocturnal animals) if you your configuration allows blocking the light from the main tank.
Where would the bulkhead screen ideally be fitted? On my system I am thinking if the octo could get through the U-tube, then it's out. That would be the only way out as I have a sponge over the actual drain, and the lid would be sealed over the overflow box (although with enough room for it to go into the overflow box).
Definitely finding enkamat needs to be cleaned at least weekly, but haven't found a better solution for what to put in the overflow box.
Sounds like you have an hob overflow?
@JingoFresh, I believe you're using an overflow box and not a drilled tank so you won't have a bulkhead. Depending upon the depth of your siphon tube and your weir (overflow box), you may be able to fashion something similar DIY. Before they made the bulkhead screens, I used (and still actually have one in place because I like it somewhat better than the official version) the removable screen from a cascade intake. If you can get to it, measure the inside diameter of the J-tube where it sits in the water.
several cross posts here - great minds ...