ViewportFrame - "Window"?

I’m currently attempting to utilise this method of “Windows” to create a somewhat ‘TARDIS’ effect.

I’m not looking to use the initial Portal module released by @Egomoose but rather to adapt the one linked above.

So far, this is what I have -

As you can see, when the Camera is rotated past a certain point, Parts from within are seen to block the Viewport.

What is a method I can use to combat this?
Note for the Demo I have removed any parts behind the actual ‘Window’.