just edit the SlackBuild and add the $PKG/install directory creation, like this
Code:
--- ./vivaldi.SlackBuild.orig 2022-08-07 18:48:22.079807000 +0200
+++ ./vivaldi.SlackBuild 2022-08-07 18:52:47.496807000 +0200
@@ -102,6 +102,7 @@
done
lynx -dump file://$CWD/${VERSION}.html > $PKG/usr/doc/$PRGNAM-$VERSION/ChangeLog
+mkdir -p $PKG/install
cat $CWD/slack-desc > $PKG/install/slack-desc
cat $CWD/doinst.sh > $PKG/install/doinst.sh
cat $CWD/douninst.sh > $PKG/install/douninst.sh
then build it manually executing the SlackBuild