Check out this insulation from McMaster Carr:
I just installed it in my motor box and was on the lake today for the first time with it. I used the 1 inch skinned self-adhesive stuff. Time will tell how the adhesive holds but it sure is quiet. I can hear the exhaust rumble better now.
2005 X2, Viper Red, MCX, OJ TZ-X5 prop, PPass, rear 750 sacs, KGB, IBS, Bennett Wake Plate, custom wake shaper
(previous) 2001 X5, 1991 TriStar 190