How can I have a second camera (3D minimap) that does not render background

I have a ship flying in space and enemies represented by red cubes, and I have second camera that renders like a minimap, as seen in this tutorial:

And in game shown as such, intended to act like a 3D radar with the player in the middle of the circle and enemies around it:

This camera also uses a culling mask, so it only renders certain GameObjects (namely the red enemies and the player in the middle), but I can't keep it from also rendering a background -it either renders skybox or a solid color
Is there a way to render a "transparent" skybox, one when overlapped with the main camera, only renders the objects but not the background/skybox?

Thank for any help/advice in advance

 

Camera component -> Clear Flags = Depth