TStretchHandle = class(TCustomControl)
  private
    procedure WMEraseBkgnd(var message: TWMEraseBkgnd); message WM_ERASEBKGND;
    procedure WMGetDLGCode(var message: TMessage); message WM_GETDLGCODE;
  protected
    procedure Paint; override;
    property Canvas;
  public
    procedure CreateParams(var Params: TCreateParams); override;
end;

procedure TStretchHandle.CreateParams(var Params: TCreateParams);
begin
  { set default Params values }
  inherited CreateParams(Params);
  { then add transparency }
  Params.ExStyle := Params.ExStyle + WS_EX_TRANSPARENT;
end;

procedure TStretchHandle.WMGetDLGCode(var message: TMessage);
begin
  { completely fake erase, don't call inherited, don't collect $200 }
  message.Result := DLGC_WANTARROWS;
end;

procedure TStretchHandle.WMEraseBkgnd(var message: TWMEraseBkgnd);
begin
  { completely fake erase, don't call inherited, don't collect $200 }
  message.Result := 1;
end;

procedure TStretchHandle.Paint;
begin
  inherited Paint;
  with Canvas do
  begin
    // рисуете что нужно -
    // где не рисовали, там будет "прозрачно"
  end;
end;