How to create a "window" through a panel ?

First of all, consider me as a total noob on Unity and C# although I already developed and sold games in the 80s and 90s (good ol'time of assembler). Ok. I am presently developing a simulation almost entirely with the UI. However, part of the screen will have a map which can turn in a 3D visualization (a bit like you do with Google Map/Google Earth). So I would like to have a kind of window in my main panel which will display the map, namely an orthographic view of a terrain from top. At any time, I will be able to press a small 3D icon in the corner of the map and access to a 3D visualization/navigation. It's pretty easy, from the few I understand, I just need to rotate and translate the camera and switch it perspective. Now, my BIG problem is how do I create this ? I need to create a window, a bit like on a smartphone case cover. I smell it has something to do with mask but I don't understand how to proceed.

Btw, there is a less elegant solution (and it looks even more complex): to use a second camera to view the terrain from top and "broadcast" the view of the camera on the panel but I don't know how to proceed as well.