Refactored to use app folder.

src/app/not-found.tsx

@@ -0,0 +1,13 @@
+'use client';
+import { Flexbox } from 'react-basics';
+import useMessages from 'components/hooks/useMessages';
+
+export default function Custom404() {
+  const { formatMessage, labels } = useMessages();
+
+  return (
+    <Flexbox alignItems="center" justifyContent="center" flex={1} style={{ minHeight: 600 }}>
+      <h1>{formatMessage(labels.pageNotFound)}</h1>
+    </Flexbox>
+  );
+}