Question Make camera ignore everything behind portal

I'm making portals and the effect is pretty smooth, but I'm having trouble with objects that are behind the portal.

I followed a tutorial and it basically works by placing a camera that imitates the movement of the player relative to the entrance, the problem is when there's an object behind the portal, and so the camera picks up on that, completely breaking the effect. How can I make the camera ignore everything that is behind the portal?

I tried making a special layer so that it would get prioritized above all else and making a flattened cube with an invisible material, but clearly that didn't do the trick.

 

Use stencil portals. There are some guides on YouTube

 

This is exactly what I was looking for, thanks a lot!