I bought some
Whispermat from Budget Marine and installed it on 5 sides of the genset box and it made a huge difference in the sound level. I also have a watermaker where the high-pressure pump makes a lot of noise and the Whispermat there also brought the sound level down from Jet-Engine levels to mere background noise. The mats are heavy and expensive but were worth every penny to me. All I neede was a tape measure, carpet knife, straight-edge and high-temperature glue for those surfaces that wouldn't stick with mere friction.