Custom attached property doesn't work like Canvas.Left

Question

In Universal Windows App flavoured Xaml, I want to create a custom panel that has a very similar attached property such as Canvas.Left (or Top, Right or Bottom). So I can simply create one according to the guide

public static readonly DependencyProperty XProperty = 
    DependencyProperty.RegisterAttached("X", typeof(double), typeof(Track), null);

public static double GetX(DependencyObject d)
{
   return (double)d.GetValue(XProperty);
}
// SetX and property wrapper omitted for brevity here

Now I can write

<c:Track>
  <TextBlock Text="1" c:Track.X="1"/>
  <TextBlock Text="2" c:Track.X="2"/>
  <TextBlock Text="3" c:Track.X="3"/>
</c:Track>

And I can then use my attached values in

public override void ArrangeOverride(Size finalSize)
{
  foreach (var child in Children)
  { 
     var x = GetX(child);
     child.Arrange(CalculateArrange(x, finalSize));
  }
}

And everything is working perfectly so far.

However when we come to an ItemsControl I can do this,

<ItemsControl ItemsSource="{Binding ListOfInts}">
  <ItemsControl.ItemPanel>
    <ItemPanelTemplate>
       <Canvas/>
    </ItemPanelTemplate>
  </ItemsControl.ItemPanel>

  <ItemsControl.ItemTemplate>
    <DataTemplate x:DatatType="Int">
      <TextBlock Canvas.Left="{Binding}" Text="{Binding}"/>
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

But if I want to do it for my own custom panel, I have to do this

<ItemsControl ItemsSource="{Binding ListOfInts}">
  <ItemsControl.ItemPanel>
    <ItemPanelTemplate>
       <Track/>   <!--Change to my panel-->
    </ItemPanelTemplate>
  </ItemsControl.ItemPanel>

 <ItemsControl.ItemContainerStyle>  <!-- need to add the attached property here -->
   <Style TargetType="ContentPresenter">
     <Setter Property="c:Track.X" Value="{Binding}"/>
   </Style>
 </ItemsControl.ItemContainerStyle>

  <ItemsControl.ItemTemplate>
    <DataTemplate x:DatatType="Int">
      <TextBlock Text="{Binding}"/>
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

Because I can't do

public override void ArrangeOverride(Size finalSize)
{
  //Children are now contentPresenters
  foreach (ContentPresenter child in Children) 
  {
    var controlWithAttribute = child.????
    //child.Content is the same as DataContext i.e. an Int

    var x = GetX(controlWithAttribute);
    child.Arrange(CalculateArrange(, finalSize));
  }
}

Any ideas what I am missing? How do I get the ItemsControl to work the same as Canvas.Left?