R – Best not sealed UI element to derive from in Silverlight

I am currently creating a few test controls. In WPF everything is fine, I can subclass e.g. Border or ScrollViewer. In Silverlight this is not possible, so I have to do a little trick (multi-targeting)…

I am deriving my test element from StackPanel and add the content as child. I could, of course, also derive from WrapPanel, Canvas….

My question is, when using this approach, which UI element in Silverlight (not sealed) is the best to use. It should interfere as little as possible with the test element. Or do I have to distiguish between elements (with width settings e.g.)..

Let me give a better example. If I want a simple TextBlock which always says "Hello" and has a red Background. In WPF, I subclass the TextBlock. This is not possible in Silverlight. There I would e.g. do this:

public class RedTextBlock : StackPanel
{
    public TextBlock WrappedBlock {get; private set;}
    public RedTextBlock()
    {
        WrappedBlock = new TextBlock
        {
            Text = "Hello",
            Background = new SolidColorBrush(Colors.Red)
        };

        Children.Add(WrappedBlock);
    }
}

I do not really like the solution, but have not found a better one. But in this case, I am not sure if the StackPanel is not way to smart for my example. Because I THINK it may influence the layout (e.g. taking as much space as possible) in opposition to a direct textblock instance.

In other cases the containing control may be a ScrollViewer, an Image, whatever.. All controls which I can't subclass "correctly" because they are sealed.

Is this approach OK, which control would be the best to subclass in this generic cases, or are there better ideas? (I do not want to go the "interface – View as Property" way, I really want to subclass existing UI element)..

Chris