Agree with @ghdprentice about the steel shot. Look for S110, which is about 1/4 of a mm in diameter, or the even smaller S70. You can mix with silica sand (the white sandbox stuff) if desired. One vendor in Kansas (link).
Lightweight packing foam may dampen ringing of the metal stand itself, but will probably not add to the overall vibration resistance. The weight from shot and/or sand should help lower the natural frequency of the combined components and stand system.

