This is a form of mass loading, a type of vibration control. Nothing special, just solid mass. Looks better than a phone book, one of the first I tried 30+ years ago. That's how old this is. You can try a book, dive weights https://systems.audiogon.com/systems/8367 lead shot, granite, butcher block, sand bed, literally anything with mass. Not all materials sound the same.
Look close, you will notice I have done this with a lot of mine.
Look close, you will notice I have done this with a lot of mine.