And now for my special blend of fairly objective advice and pure opinion!
It will escape. They are not "fairly smart", they are extremely intelligent. All rocks will need to be secured in place and to each other. The lid cannot have any opening large enough for the beak to fit through. If the beak fits, the entire animal fits. No other animals can be kept with it or they will be food.
And now for my opinion. Octopi are not pets. They should not be pets. They are intelligent to the point at which sentience is debated. They may be self aware and capable of complex emotions such as boredom, depresson, anger, happiness, etc. None of this is proven, but there is evidence. No animal that may be self aware should be kept in a cage.
Just my two cents.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef