You could create a script to modify the top level folder permissions to a different group that those users aren't a member of. That way the drive mapping will stay intact but they just won't have access to the folder. Run the script at the times you indicate.
Because the IPKG stuff does not work for me on a DS218+, because the ipgk installes a wrong version of ncurses (needed by ncdu), the most easy way for me is using "docker".
For example: https://hub.docker.com/r/bernadinm/ncdu (there are many more)
install the container
docker run -it -v /:/mount bernadinm/ncdu /mount
and you have full functional "ncdu" ...