Portal Error

Hi guys!
I’ve been trying to make an animation in the style of the game Portal, and everything has been working so far, but when i try to make the portal view, this happens:

It should be showing a view from the portal, but it’s not, its just showing a platform.
Here’s a diagram:

the triangle is what is seen through the portal. as you see, there’s the platform in the view. This is what I want:

I kinda want to mask out anything behind the portal, so that kind of error doesn’t happen.
I’ve tried adding a holdout, but that just makes everything invisible.
And I can’t do booleans, because it’s a collection we’re dealing with.
It’s kinda complicated, but is there a way?