I put a tractor muffler on mine. It doesn't really reduce the volume, but changes the sound and gets rid of some of the more annoying notes.
I had more overall success lining the generator compartment with Dynamat, creating a floor for the generator compartment out of stall mat, using the leftover stall mat to make better mounts, and a few other little things.
Despite all the things, it's still annoying and still loud. If I ran it very often, I'd consider replacing it with the new inverter model. That being said, it's way better than when I bought it.