I recently tried a "ghost" tube. I had a clear acrylic tube left over from a previous freshwater tank. These tubes are used with ghost knifefish and eels. Neither see well, and will hide in these clear tubes. They think they are in something that is solid and hiding them as they only feel the tube around them and associate it with safety. The octo played with the 8 inch tube for a while but wasnt really sure what to make of it. It would go to one end and then extend its tentacles through to the other end. Then would go do the same fromt he other end. In the end, since it was big it kind of lost interest in it. It never tried (while I was watching), to pass through it.
Though the octopus is big (peach sized bell and 8-10" arms) the ghost tubes size was too cumbersome due to rock, etc for him to do much with it. The ghost tube is 8" long and about 1-1/2" to 2" in diameter.