Welcome to TONMO, the premier cephalopod interest community, and birthplace of #WorldOctopusDay and #CephalopodAwarenessDays. Founded in 2000, we are a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up. You can also become a Supporter for just $50/year to remove all ads and enjoy other perks. Follow us on Twitter for more cephy goodness.
IME, the never pick the spots we make "perfect" for them as a den. Big caves are not usually their choice, they will seek an opening just big enough for their mantle with no room for predators. Some of mine have chosen places with a second entrance/escape where others have preferred a single opening. You might also think about a few small light rocks or shells (thumb nail to thumb sized) to be used as a door.