Hello Paul,

My suggestion for achieving your requirement is using DrawFilter. In the DrawFilter class implementation you can create a pen with color and width as you need and draw line only on top of the Panel. 

public class DrawFilter : IUIElementDrawFilter
    {
        Pen borderPen = new Pen(Color.DarkGoldenrod, 15);
        public bool DrawElement(DrawPhase drawPhase, ref UIElementDrawParams drawParams)
        {
            // If the element being drawn is a ControlUIElementBase
            // we're interested in drawing its borders only.
            Rectangle elementRect = drawParams.Element.Rect;
            drawParams.Graphics.DrawLine(this.borderPen,
              elementRect.Location,
              new Point(elementRect.Right, elementRect.Top));
            // Return true to prevent the element from drawing
            // its borders normally.
            return true;
        }
 
        public DrawPhase GetPhasesToFilter(ref UIElementDrawParams drawParams)
        {
            if (drawParams.Element is ControlUIElementBase)
                return DrawPhase.BeforeDrawElement;
            return DrawPhase.None;
        }
    }
 

I have attached a sample application that uses this approach. Please test it on your side and let me know if I may be of any further assistance. 

Sincerely,
Teodosia Hristodorova
Associate Software Developer

0602.Top_Border_UltraPanel.zip