Flutter 将自定义容器附加到屏幕底部

我有一个RegisterScreen，我有一个图像作为背景，在所述图像上，我想要一个具有玻璃形态效果的容器，但我的自定义GlassMorismContainer小部件不会附加到屏幕底部

class RegisterScreen extends StatelessWidget {
  const RegisterScreen({super.key});

  @override
  Widget build(BuildContext context) {
    return Container(
      constraints: const BoxConstraints.expand(),
      decoration: const BoxDecoration(
          image: DecorationImage(
              image: AssetImage("lib/assets/images/rainforest_Placeholder.jpg"),
              fit: BoxFit.cover)),
      child: Stack(
        children: [
          Align(
            alignment: Alignment.bottomCenter,
            child: GlassmorphismContainer(
                width: MediaQuery.of(context).size.width, height: 350),
          ),
        ],
      ),
    );
  }
}

class GlassmorphismContainer extends StatelessWidget {
  final double width;
  final double height;
  final Widget? child;

  const GlassmorphismContainer(
      {super.key, required this.width, required this.height, this.child});

  @override
  Widget build(BuildContext context) {
    if (child == null) {
      return Stack(
        children: <Widget>[
          ConstrainedBox(
            constraints: const BoxConstraints.expand(),
          ),
          Center(
            child: ClipRect(
              child: BackdropFilter(
                filter: ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
                child: Container(
                  width: width,
                  height: height,
                  decoration: BoxDecoration(
                      color: Colors.grey.shade600.withOpacity(0.5)),
                ),
              ),
            ),
          ),
        ],
      );
    }
    return Stack(
      children: <Widget>[
        ConstrainedBox(
          constraints: const BoxConstraints.expand(),
        ),
        Center(
          child: ClipRect(
            child: BackdropFilter(
              filter: ImageFilter.blur(sigmaX: 10.0, sigmaY: 10.0),
              child: Container(
                width: width,
                height: height,
                decoration:
                    BoxDecoration(color: Colors.grey.shade600.withOpacity(0.5)),
                child: child,
              ),
            ),
          ),
        ),
      ],
    );
  }
}

我已经try 过使用Column和MainAxisAlignment.end以及上面的方法和Stack和Align