How to get ListBox ItemsPanel in code behind

I have a ListBox with an ItemsPanel

<Setter Property="ItemsPanel">
    <Setter.Value>
        <ItemsPanelTemplate>
             <StackPanel x:Name="ThumbListStack" Orientation="Horizontal" />
        </ItemsPanelTemplate>
    </Setter.Value>
</Setter>

I am wanting to move the Stack Panel along the X-axis using a TranslateTransform in code behind.

Problem is, I can't find the Stack Panel.

ThumbListBox.FindName("ThumbListStack")

Returns nothing. I want to use it in:

Storyboard.SetTarget(x, ThumbListBox.FindName("ThumbListStack"))

How do I get the Stack Panel so I can then use it with the TranslateTransform

Thanks