What about a hot tub? I used to work at a place that kept baby Elephant Seals in a converted hot tub. It was a rehabilitation center for seals and sea lions. Someone donated the hot tub to the center. If 15 elephant seals (ok, it was a little bit crowded at times...), then surely one cuttlefish could fit.
Wow, that is my actual plan for sepia apama. I've had to nix the idea of having a trapdoor bridge spanning it like in "You Only Live Twice" though. What if the victims, er, guests had pennies in their pockets? Wouldn't want any copper near my cephs.
Hey, how about if we make a real big ceph tank...I mean, really, really big...so we don't have to do water changes at all !
Now, it'll have a rocky substrate, some sandy areas, other animals so we don't get bored looking at just the cephs, and different thermal gradients so the animals can pick their own climes.