How to implement "Under Construction" site?

Dear community,

how could we realize an "under construction" site for our dashboard running on NW2004s? Visitors enter the dashboard by given URL. From time to time we now want to close the dashboard for visitors and want to show a site instead that informs the visitor that we are currently maintaining the site.

How to do that? Of course, without touching each and every WebTemplate. I know how one could do that in Apache webserver. But in NW2004s, I have no idea yet.

Any hints welcome. May be this is a setting in Custominzing? Did not find any.

Regards,

Andreas