The catacombs are a no-go, and Luther raised an excellent point about the dangers of locks that can't be unlocked on a timer. If there was a fire - which, let's be real, is very likely with this crowd - the people in those rooms would be roasted.
So here's my thought.
What if we pooled our funds and rented a first-floor apartment? I could outfit it completely for everybody that wants to lock themselves away for the good of the community, and we can assign trustworthy people to guard it and let people out if an emergency happens.