Let TextBox stretch to fill width in StackPanel

I have a StackPanel in my WinRT C# Metro app that I want to use as container for a label/value pair (TextBlock and TextBox) like this:

<StackPanel Orientation="Horizontal" Width="400">
    <TextBlock Text="Label" Width="150" />
    <TextBox Text="Value" />
</StackPanel>

Now what I want is to let the TextBox automatically fill the remaining horizontal space of the StackPanel. Is this possible in any way? HorizontalAlignment/HorizontalContentAlignment don't work.

I know that an alternative would be to define a Grid for that. The problem is that I have this construct several times and want to use it in a Style definition. I don't want to define the Grid's definition with rows and columns x times...

Perhaps the alternative would be to define a custom user control, but I hoped there would be an easy possibility to get the TextBox to stretch.

Unfortunately the alternative (DockPanel) isnt available for Metro. You could try a WrapGrid, but I dont know if it'll solve your problem (Ive never used it).

The only real way of doing this is using a Grid as you described:

<Grid Width="400">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition />
    </Grid.ColumnDefinitions>
    <TextBlock Text="Label" Width="150" />
    <TextBox Text="Value" Grid.Column="1" />
</Grid>