Objects visible only through portals with no material change

Hello everyone,

I am struggling quite a bit with an effect I want for my game.
The effect itself is not that complicated and a practical way of doing it is even described here.
Using Stencil buffer - Development / Rendering - Epic Developer Community Forums
The goal is for chosen objects to be seen only through a portal or window, but not outside. Or vice versa, objects visible outside but not inside the window.

Again the effect I did manage to do via multiple methods, including the stencil buffer I just linked. There is a major constraint though. It is imperative for me not to change the look and material of any object affected by this effect. None of the methods I found complied with this.

That’s why I tried a bunch of things using a post process material to calculate the depth of the affected object versus that of the window to create a mask but to no avail.

I’d appreciate it a lot if someone can nudge me in the right direction.
I should also add I am only using blueprints.

Thanks in advance for any reply.