Vista - Using clip on item of unknown size

I have a button design, which for the sake of simplicity we'll say is a
rectangle with a drop shadow.

I have control templates for up/down/disabled etc.
All OK so far.

Now I want to implement a button where the bottom edge is hidden (so the
lower edge of the drop shadow is not visible.

If I (for example) create a button as a rectangle Width="200" Height="100"
and dropshadow size 5, I can then apply a Rectangle.Clip with a Rect of
"0,0,210,100" and all the drop shadow of the lower edge is clipped off. This
is exactly what I want to see.
But I'm trying to produce the template without hard coding the button size.
So if I could do this with Margins somehow (or using a thin strip on a grid)
I could make it work for any size button it got used with.

Anyone got any ideas ?

Thanks in advance


Griff
(trying to make an industrial UI with XAML/WPF/c#)