Canvas position based on another element - wpf

Question

Basically I want a Canvas to act like a StackPanel. So how do set the Canvas.Top="" based on Canvas.Bottom of another element?
Background: Trying to make an Expander that when expanded it will go over other elements. Figured using a Canvas ZIndex would be the best way to do this. So I created this:
<StackPanel Orientation="Vertical" >
<Canvas Height="{Binding ActualHeight, ElementName=MyExpander}">
<Expander Panel.ZIndex="1" Name="MyExpander" Header="Header" >
<StackPanel Background="LightGray">
<TextBlock Text="Some text" />
<TextBlock Text="Some text" />
<TextBlock Text="Some text" />
<TextBlock Text="Some text" />
<TextBlock Text="Some text" />
<TextBlock Text="Some text" />
</StackPanel>
</Expander>
<StackPanel Panel.ZIndex="0" Canvas.Top="20" Margin="0,5,0,0">
<Button Content="Button1" />
<Button Content="Button2" />
<Button Content="Button3" />
</StackPanel>
</Canvas>
</StackPanel>
Now this works perfectly the problem is that Canvas.Top="20" is hardcoded into the XAML. So that means if the font gets increased (the user increased font sizing in Windows) then part of the StackPanel will be under the Expander. I tried this:
Canvas.Bottom="{Binding (Canvas.Bottom),ElementName=MyExpander}"
The issue being is that value for for Canvas.Bottom for MyExpander is NaN so that isn't going to work.
FYI if there is a better way to do the Expander expands over top of elements I am open to that as well.
thanks

Answer 1

Well this doesn't seem like the greatest answer but it worked for me.
foreach (var exp in MyCanvas.Children)
{
Expander ep = (Expander)exp;
double h = ep.ActualHeight;
Canvas.SetTop(ep, y);
y = y + h + 20;
}
Should note it goes without saying that foreach loop is a bit more complicated due to correct parsing of the objects in the MyCanvas