It could be an artifact that your PC finds on the nautilus ship. That way you don't need a high level spellcaster and you would have it right from the start of the game. And Larian wouldn't need to do redundant work by creating "regular" camp environments in addition to the pocket dimension. Over time the "camp" gets more elaborate as your party grows. Maybe you find spells or runes or something that increases the artifact's power which makes it get extra features.
