internal class CustomControlDesigner : ParentControlDesigner
{
    public override void Initialize(IComponent component)
    {
        base.Initialize(component);

        if (this.Control is MyCustomControl)
        {
            EnableDesignMode(((MyCustomControl)this.Control).ContainerPanel, "ContainerPanel");
        }
    }
}

[Designer(typeof(CustomControlDesigner))]
public partial class MyCustomControl : Control
{
    private Panel _containerPanel;

    // Attribute for code generation when a control is added at design time
    [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
    [Browsable(false)]
    [EditorBrowsable(EditorBrowsableState.Never)]
    public Panel ContainerPanel
    {
        get { return _containerPanel; }
    }

    public MyCustomControl()
    {
        _containerPanel = new Panel();
        // Panel location, size, etc.
        this.Controls.Add(_containerPanel);
    }
}