The following draw filter will draw a red rectangle instead of the normal grab handle. You can modify it slightly to draw your own grab handle:

public class DrawFilter : IUIElementDrawFilter
{
 bool IUIElementDrawFilter.DrawElement( DrawPhase drawPhase, ref UIElementDrawParams drawParams )
 {
  // Draw the custom grab handle here instead of the red rectangle.
  drawParams.Graphics.FillRectangle( Brushes.Red, drawParams.Element.Rect );

  return true;
 }

 DrawPhase IUIElementDrawFilter.GetPhasesToFilter( ref UIElementDrawParams drawParams )
 {
  if ( drawParams.Element is GrabHandleUIElement )
   return DrawPhase.BeforeDrawForeground;

  return DrawPhase.None;
 }
}