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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.