I would notch the plexiglass for the cord. We cut off the plugs,threaded the wire through, and put new plug ends on our Koralia's for one tank. What a mess when we had to replace the unit, never again. As long as the notch is relatively tight (SAND THE EDGES or it will cut the wire), it has not been a problem in our other configurations.
Love the video with the arm investigating out the side while she looks innocently observing out the front door.
The blue leg "body" may have been a shed rather than the actual critter. We thought we lost our little lobster once. Neal informed me that it had died a few months after we put it in its tank. I was examining the carcass and noted the separation between the tail and the body and that there was no meat. I kind of grinned, said, NOT and went looking for it (still with us today and continues to shed every couple of months). Very hard to see this with the little blue legs though.
I just zip the zipper around the "pole" side of the Koralia. This makes it easy to remove and clean the netting and has eliminated any issue with octopus arms.
Here is a post
that CaptFish and I put together for animals we have kept successfully with octopuses.
You can put leathers and mushrooms in the tank. Polyps are not easy to identify for OK and not OK. I have one set that the octos will literally sit on but have had to remove others that look similar because the octo will touch and jerk away. I have propagated the safe species in all three tanks I keep for octos and have not bothered trying others. I've not tried Xenia with an octopus.
Gorgonians are fine but keep in mind that octopuses do not go around things so placing one in the current and away from an octopus path can be a hard location to find.
Typically, anything I try is on its own rock so that I can remove it easily or move it if it looks like the octo will cause it harm by continually crawling on it. That being said, I have a video of Maya moving a gorgonian across the 4' tank and planting it at her den door.
Snails of any sort should be fine. Like hermits, if they are eaten at all it is usually only until they know that easier food will be provided (again my vulgaris was the exception). Snails with an operculum (trap door made of material similar to your fingernail) seem to survive well. I have looked for some without to use as food but there are only a few saltwater species (and the ones I found are actually brackish) that don't have one. The one place I have seen them, and not often, is the Asian food market. Should you buy some, try a couple in fresh and a couple in saltwater. If the saltwater does not kill them after a day, they should be fine.
I always keep a red serpent star in my octo tanks (and usually some other serpent). The red/orange animals are more active than most, often live in the octopus' den (sometimes making it easier to find the octopus) and are less shy of humans. I also like to keep a couple of thorny stars in the bigger tanks since these are diurnal, can be brightly colored and seem to do well. I have read that they are algae eaters but mine have all clearly been carnivorous.
Any kind of clam seems to do well and may or may not become food (not likely with a dwarf). I get them from the seafood market, leave them overnight in tank water with one water change (they need the oxygen of a full water change) to clean out whatever yucky water they came with and place them on the sand. They will help a little with water quality and don't make the mess of other mussels if they die (do watch and remove if dead though).