If a space craft was built by a cephalopod race, what would it look like? The Cephalopods in question are not humanoid at all and look like large octopuses.
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Our earth cephalopods have two distinct branches, as described in this article. They are always aquatic, so your intelligent cephalopods would likely also be. Creatures on the Nautiloidea side have (often beautiful) external shells. On the other hand, the Coleoidea side are the ones who have the tentacles, which might serve as a replacement for hands with opposable thumbs.
Octopuses are the most intelligent invertebrates, having intelligence on par with cats (who I personally believe are smarter than they let on, but that's a tale for another day). This article was very informative, and leads me to believe that if an aquatic civilization were to ever arise on our planet, the octopuses would firmly lead the way (not dolphins or whales as we tend to believe).
An interesting fact about octopuses is that they are able to edit the messages sent from their DNA, and some scientist speculate that this ability directly contributes to their intelligence. This ability is limited to the Coleoideas (on our planet), but maybe not on the one where Octopuses became the dominant species.
Octopuses might have slithered to the top of their world's food chain by perfecting their ability to direct the evolution of their shelled cousins to provide tools, building materials and so on. This kind of deliberate tinkering would probably enhanced their ability to reason and could well have allowed them to achieve advanced civilization without ever harnessing fire or discovering the wheel.
So in this scenario, Octopuses would either engineer a gigantic nautilus (along with other types of shellfish, for inclusion in a spacecraft), or have engineered massive nautilus whose shells would be combined in some way to build the spacecraft.
To a species which is entirely nautical, their first "space" exploration would be of the world outside their oceanic home. I think that their awareness of actual space would only come to be after they leave the ocean in their first proto-spacecraft.