Rendering completely separate camera (created at runtime) on top of "default"

So, in Unity, it is a fairly easy way to setup multiple cameras in overlay:

Now, I know I can render to RT and draw a texture on screen, but this is very expensive to do full-res RT. Assuming I’m limited to BP, is there a way to “spawn” a completely new camera and force it render on top of existing one for specific period of time, ovelaying and occluding previous one?