Getting round the fencepost principle in a WPF ItemsControl

I've got an ItemsControl with an item DataTemplate that includes a Margin value of 0,0,0,20 to add a 20-pixel gap between each item. The problem is, when this control is scrolling, there's a spare 20px after the final item caused by this margin being applied to the final item.

How do I remove/clip this gap at the end, or ensure it is not added in the first place? ie, apply a 20px gap between items but not at the start or end of the list?

On the ItemsPanelTemplate, set the Margin property to "0,0,0,-20".

Example:

<Grid Background="Black">
    <ItemsControl ItemsSource="abcdefg" Background="White" VerticalAlignment="Top">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Border Margin="0,0,0,20" Background="Silver">
                    <TextBlock Text="{Binding}" Margin="5" />
                </Border>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
        <ItemsControl.ItemsPanel>
            <ItemsPanelTemplate>
                <StackPanel Margin="0,0,0,-20" />
            </ItemsPanelTemplate>
        </ItemsControl.ItemsPanel>
    </ItemsControl>
</Grid>