Taping behind the seat just for dust control is a great idea! Be warned that whatever you use, be it tape or Dynamat, it will have to periodically come out so that you can do an annual bevel box oil change. It really is much easier that way.
As I changed out the rear sprocket for the Super Max refurb, the rear noise isn't an issue for me. Loosening the belt tension helps to kill the bevel box whine. I do think that using Dynamat on the transmission housing as well as the driveshaft cover would be a nice touch.
