Top Left Highlight Ribbon Positioning

I'm trying to create a high-light ribbon that appears diagonally across the top-l开发者_运维百科eft of my grid.

I just can't seem to figure out the best way to get the layout - especially seeing our application can be resized.

As an example of what I'm trying to achive, see the top-left red highlight with "Best Choice" written in it: http://www.photoshopstar.com/wp-content/uploads/2009/11/stylish_advert_21.jpg

How can I get this sort of layout in WPF?