How to make Delphi TFrame background transparent

unit Knob;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, VrControls, VrWheel;

type
  TfrKnob = class(TFrame)
    vwKnob: TVrWheel;
    lPos: TLabel;
    procedure CreateParams(var Params: TCreateParams);override;
    procedure PaintWindow(DC: HDC); override;
  private
    { Private declarations }
  public
    constructor Create(AOwner:TComponent);override;
    { Public declarations }
  end;

implementation

{$R *.dfm}

constructor TfrKnob.Create(AOwner: TComponent);
begin
  inherited;
  Brush.Style := bsClear;
end;

procedure TfrKnob.CreateParams(var Params: TCreateParams);
begin
  inherited;
  Params.ExStyle := Params.ExStyle or WS_EX_TRANSPARENT;
end;

procedure TfrKnob.PaintWindow(DC: HDC);
begin
// keep this comment or Delphi will loose this method implementation
end;

end.