This is the GUI created by the following script example

# Central Start

# Python start script goes here
print 'Central Start'

selectProductOverlay = CreateSelectProductOverlay(GetCustomPage('Select product').handle,'ProductType')

# Central Stop

# Python stop script goes here

try:
  selectProductOverlay.Dispose()
except:
  pass

# CreateSelectProductOverlay

def CreateSelectProductOverlay(hwnd,nameOfExternalData):
  import CLR
  import CLR.System.Windows.Forms as WinForms
  from CLR.System.Drawing import Size, Point, Pen, Color, SystemColors
  from CLR.System.Drawing import ContentAlignment, Font, FontStyle, Image
  from CLR.System import Int32, Decimal

  # Load custom assemblies
  from CLR.System.Reflection import Assembly
  Assembly.LoadFrom('OverlayPanel.dll')
  from CLR.Tordivel import OverlayPanel

  # WinForms.ComboBox connected to an ExternalData tool
  class SelectProductCombo(WinForms.ComboBox):

    def __init__(self):
       pass

    def Init(self,nameOfExternalDataTool):

      self.ExternalData = nameOfExternalDataTool
      self.SelectedValueChanged += self.SelectedValueChangedHandler
      self.SetProductComboBox()

    def SetProductComboBox(self):
      plist = GetResultValue(self.ExternalData+'.Dictionary')
      list = []
      for element in plist:
         list.append(element)
      print list
      list.sort()
      for product in list:
        self.Items.Add(str(product))
      self.Text = GetValue(self.ExternalData+'.Value')

    # eventhandler
    def SelectedValueChangedHandler(self, sender, args):
      SetValue(self.ExternalData+'.Value',self.Text)


  class SelectProductOverlay(OverlayPanel):
    def __init__(self,nativeParentHandleAsInt32):

      # This enables the panel to follow resizing of the parent
      self.Dock = WinForms.DockStyle.Fill
      
      self.Title = "Washer Inspection"
      self.LabelText = "Select product number"
      self.FontName = "Microsoft Sans Serif"

      # Create the profile label
      self.InspectionName = WinForms.Label()
      self.InspectionName.Parent = self
      self.InspectionName.BorderStyle = WinForms.BorderStyle.None
      self.InspectionName.Font = Font(self.FontName, 22, FontStyle.Bold)
      self.InspectionName.Size = Size(300, 50)
      self.InspectionName.Location= Point ((self.Width/2-self.InspectionName.Width/2),30)
      self.InspectionName.Text = self.Title
      self.InspectionName.TextAlign = ContentAlignment.MiddleCenter

      # Create the inputLabel (after the textBox to avoid hiding)
      self.SelectProductLabel = WinForms.Label()
      self.SelectProductLabel.Parent = self
      self.SelectProductLabel.Font = Font(self.FontName, 16, FontStyle.Bold)
      self.SelectProductLabel.Size = Size(250,25)
      self.SelectProductLabel.Location = Point((self.Width/2-self.SelectProductLabel.Width)/2,120)
      self.SelectProductLabel.Text = self.LabelText
      self.SelectProductLabel.TextAlign = ContentAlignment.MiddleCenter

      # Create the select product combobox
      self.SelectProduct = SelectProductCombo()
      self.SelectProduct.Parent = self
      self.SelectProduct.Size = Size(150,30)
      self.SelectProduct.Location = Point((3*self.Width/2-self.SelectProduct.Width)/2,120)
      self.SelectProduct.Font= Font(self.FontName, 16, FontStyle.Bold)
      self.SelectProduct.Init(nameOfExternalData)
      
       # Register event handler for Resize
      self.Resize += self.ResizeHandler

    def ResizeHandler(self, sender, args):
    # Handles the Resize event

      self.InspectionName.Location = Point((self.Width/2-self.InspectionName.Width/2),30)
      self.SelectProduct.Location = Point(self.Width/2+5,120)
      self.SelectProductLabel.Location = Point(self.SelectProduct.Location.X-(self.SelectProductLabel.Width+10),120)

  return SelectProductOverlay(hwnd)