I have a Wildwood open back banjo. And it's pretty bright sounding as these banjos go. I have tried stuffing rags, towels, even small pillows in the back. And I never was satisfied with the results. The banjo just sounded like it had been stuffed with rags.

The idea, according to my banjo picking pal in the band I play in is to make the banjo sound more tubby and percussive. It works, sort of, on his antique, which has a TuBaphone tone ring in it.

As to the thought that the old-timers put rags in their banjos, I rather have my doubts. Uncle Dave Macon played several banjos, one of which was a resonator banjo.

If you need a mute, the rags in the back of the banjo works. If you want a tubby sounding banjo, buy a tubby sounding banjo.
Jim