Giving custom shape to showcase widget's tooltip in Flutter

Question

In flutter app, I am using flutter show case view dependency

class CustomShowCaseWidget extends StatelessWidget {
  final Widget child;
  final String description;
  final GlobalKey globalKey;
  final double radius;
  const CustomShowCaseWidget(
      {Key key,
      @required this.child,
      @required this.description,
      @required this.globalKey,
      @required this.radius})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Showcase(
        key: globalKey,
        showArrow: false,
        description: description,
        disableAnimation: true,
        shapeBorder: const CircleBorder(),
        radius: BorderRadius.all(Radius.circular(radius)),
        child: child);
  }
}

For now, the tooltip background is a rectangle which is rounded at corner and it appears in the vicinity of the showcased item. But I want it to be of a certain shape, in my case, left bottom quarter of a circle which is stuck to the very right of the screen at 3/10th height of the device for every showcased item for it may be at the top or very bottom of the screen. Can I do this with these params? :

 const Showcase({
    required this.key,
    required this.child,
    this.title,
    required this.description,
    this.shapeBorder,
    this.overlayColor = Colors.black45,
    this.overlayOpacity = 0.75,
    this.titleTextStyle,
    this.descTextStyle,
    this.showcaseBackgroundColor = Colors.white,
    this.textColor = Colors.black,
    this.scrollLoadingWidget = const CircularProgressIndicator(
        valueColor: AlwaysStoppedAnimation(Colors.white)),
    this.showArrow = true,
    this.onTargetClick,
    this.disposeOnTap,
    this.animationDuration = const Duration(milliseconds: 2000),
    this.disableAnimation,
    this.contentPadding =
        const EdgeInsets.symmetric(vertical: 8, horizontal: 8),
    this.onToolTipClick,
    this.overlayPadding = EdgeInsets.zero,
    this.blurValue,
    this.radius,
    this.onTargetLongPress,
    this.onTargetDoubleTap,
    this.tipBorderRadius,
  })