How to create a portal effect (portal doesn't need to be usable)?

Hi,

I am about to import my models from Maya. In my scene is a portal which, within the frame, I would like to have a different environment visible that pans and rotates along with the camera. As it says in the question, the portal won’t be usable as I’m not making a playable level, just a diorama. Below is a sketch of the sort of thing I’m talking about. Any direction would be super helpful. Thanks.