I recommend live rock. This gives you a variety of life and it looks more visually appealing than a chunk of plastic. None of my rock is glued in place. Make sure you have some small loose rocks that the octopus can move around to make a cave. When it comes to tank size bigger is better since it allows the octo to jet around on occassion. Use an oversized wet/dry filter, mine is enough for a 200-300 gallon tank. Use a large good skimmer too. I don't use any powerheads, instead I use a large external pump and split the return. This makes sealing the top easier, I have pics posted on here of how I did it.
Get the tank up and running, add lots of hermit crabs, snails and a couple brittle stars. This will provide food and a cleanup crew if he desn't eat them. I have cowries, cleaner shrimp and urchins in my tank also. Keep in mind that anything you put in the tank might get eaten.