Silverlight: template binding

I am trying to create a perfect circular button, so I created a template from a standard button control using Expression Blend. A regular button has a BorderThickness property of a Background object (border type) bound to a template using a template binding. If I wanted my button border to be customizable at design time, how would I bind it to the template? This will prevent me from binding the width of the circle strokes to the BorderThickness property of the templates. I can bind the Border BorderThickness object to the BorderThickness property of templates using template binding, but I don't know how to set the border to always be a circle.