World Space UI scale to fit screen

Hi,

I'm creating a UI for the Rift, so as far as I know this means Screen Space is out and I must use World Space canvas.

I want to have a dark, slightly transparent background overlay the screen when menu is displayed. But I can't get my canvas to fit the screen properly. I'm currently setting RectTransforms sizeDelta property to match Screen.width/Screen.height, but this doesn't seem to work. I've tried placing the canvas in front of camera , and played with various scale and pos Z...

World Space UI scale to fit screen