Might not totally apply here for a portable generator, but conveniently came across this the other day shopping for acoustic panels for my media room..might give some ideas:
www.acoustimac.com

Customer Photos - Russell - RockWool Generator Application
I needed something to "sound" insulate a set of mufflers for my Generac and this stuff fit the bill. Bought one (12) pack of the 1" wool and used two layers, easy to cut with hand shears. Was advised
