I think you would want non magnetic metal or XXXX, but that alone might cause vibrations in other places. I would also damp the inside of the cab with Soundcoat, or Sorbothane, and maybe some Marigo dots on the board, chips, and caps. I mass damp my cabinets on the outside with a brick, shakti, brass, bag of sand, etc. with a piece of damping between the top of cab and weight(except for the sand), i've used bubble wrap in that application also.
