How to go about drawing perimeter of RTS-Style selection rings?

Hey,

I'm currently developing an tower defense with heavy rts elements. Whenever the player selects a structure(s), a range indicator is displayed - like so:

Once you start selecting a lot of structures, it can get pretty messy. I've seen a few games out there that subtract everything in the center of the selection ring, leaving just the border around all the selected units/structures. Is there a custom shader for something like this? I was thinking about trying to implement depth masks, but I'm not sure if it's the best way to go. I've done countless searches, but haven't really found much on this topic.

Thanks