How to create a selection ring like in Starcraft 2?

While building an RTS, how would I go about creating a selection ring under a specific unit, like in Starcraft 2?

What I mean is the green ring here: http://www.sc2mapster.com/media/attachments/22/523/Screen_Shot_2011-12-03_at_4.59.49_PM.png

Right now I have a cylinder (a disc didn't seem to work, somehow) but it's solid. See http://www.babylonjs-playground.com/#1WZASW#34

Any pointers?