Working portal system in Unity. Camera can seamlessly move through them.
The portal object has a MonoBehaviour that renders a cached camera before rendering itself using MonoBehaviour's "OnWillRenderObject", this creates a chain reaction of layers until a fixed amount is reached.
This amount can be anything from 1 to 16, or if you feel like destroying your memory you could ofcourse set it higher than that.

It is also not limited to only 2 portals. You can create as many pairs as you want, just link them together in their inspectors. However they seem to sneakily steal each other's temporary buffers when having more than 2 pairs.

You can throw any rigidbody through the portals, in the example scene is a ball you can roll through it. Only it slowly becomes an egg after a few times, weirdest glitch ever. If you make Portal a rigidbody and throw it through it's connecting portal, your PC might explode.

Can be a bit glitchy when using certain rotations, still working on that