I pulled out a Carboy that I have used for a few years. It was a bit grimey with hop yeast and trub particles in there. I have no bottle brush so I had to throw in a wad of paper towels with a little star San.
Holding the Carboy sideways and using a little centrifugal motion to swirl the paper towel around at my desired location worked great! And it doesn’t scratch it up like a bottle brush.
I’m sure it’s not revolutionary but I figured I’d pass it on in case anyone found it useful!