Minimap - drawing the main Camera's projection zone?

Apologies for the vague title - to explain, I’ve got 2 camera’s on my scene. One acting as a minimap in the bottom right corner, and then of course, the main camera for the player to control.

The game is a top-down RTS; and I’m trying to set up a system where the viewport of the main camera can be rendered on the minimap camera; ala, drawing a box around the map, where the main camera can see.

Any help would be much appreciated!

Use the ViewportPointToRay method on your main camera to get a ray for each corner of the screen; then project the rays however far you like, eg until they got something, or until they reach the ground plane.

The Camera.ViewportPointToRay documentation has an example of casting a ray through a pixel.