The only true way to silence the bench is what I have done and it is not a complete problem solver but it helps.
By using a wooden prop or plastic composite you have to file the angle of the prop a bit, as well as cut the notch out for the carb. By changing the pitch of the blade you reduce the noise that it makes. The noise of the blade by far is much worse than the pipe.
When removing material from the prop dont take off to much do a little at a time..