You really don't need anything too complicated with moving parts. Just take a 1 liter plastic soda bottle. Tie a piece of fishing line around the neck of the bottle. Weigh it down with a few rocks. Put something really tempting like a piece of silverside, or fresh crab leg or a chunk of lobster inside the bottle. Drop the bottle down after lights out near the place where it hides out. Shouldn't take too long for it to smell the treat. Since octopi have no bones, they can slip into very small openings. It should have no trouble with the open neck of the soda bottle. Once it goes inside for it's treat just pull it up. I had a hitchhiker like that once. I set up its own tank for it and it did quite well for 2 years! Most octopi don't live very long. I went to the local oriental grocery store. They have lots of different raw frozen fish products, clams, oysters, etc. Octopi LOVE A CHALLENGE.
Good luck!