layout - How do I make a WPF data template fill the entire width of the listbox?

Question

I have a ListBox DataTemplate in WPF. I want one item to be tight against the left side of the ListBox and another item to be tight against the right side, but I can't figure out how to do this.

So far I have a Grid with three columns, the left and right ones have content and the center is a placeholder with it's width set to "*". Where am I going wrong?

Here is the code:

<DataTemplate x:Key="SmallCustomerListItem">
    <Grid HorizontalAlignment="Stretch">
        <Grid.RowDefinitions>
            <RowDefinition/>
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition/>
        </Grid.ColumnDefinitions>
        <WrapPanel HorizontalAlignment="Stretch" Margin="0">
            <!--Some content here-->
            <TextBlock Text="{Binding Path=LastName}" TextWrapping="Wrap" FontSize="24"/>
            <TextBlock Text=", " TextWrapping="Wrap" FontSize="24"/>
            <TextBlock Text="{Binding Path=FirstName}" TextWrapping="Wrap" FontSize="24"/>

        </WrapPanel>
        <ListBox ItemsSource="{Binding Path=PhoneNumbers}" Grid.Column="2" d:DesignWidth="100" d:DesignHeight="50"
     Margin="8,0" Background="Transparent" BorderBrush="Transparent" IsHitTestVisible="False" HorizontalAlignment="Stretch"/>
    </Grid>
</DataTemplate>

Answer 1

I also had to set:

HorizontalContentAlignment="Stretch"

on the containing ListBox.