https://slackbuilds.org/repository/15.0/network/rclone/ より https://slackbuilds.org/slackbuilds/15.0/network/rclone.tar.gzをダウンロード＆展開し、rclone自体のソースをダウンロード＆rclone.SlackBuildを実行。
tar xf rclone.tar.gz
cd rclone
wget https://github.com/rclone/rclone/releases/download/v1.63.1/rclone-v1.63.1-linux-amd64.zip
sh rclone.SlackBuild
installpkg /tmp/rclone-1.63.1-x86_64-1_SBo.tgz