Sometimes i will find myself in a public space that is acoustically interesting. It is often a parking garage stairwell or marble building lobby. This time of year is probably the best time to attempt this kind of thing - a small group that just locates themselves in such a space and simply starts singing. I am sure others have done this, and like I said, this time of year, people are likely to be more forgiving (especially if the chorus is good!!) … I would definitely consider being a part of something like this.