The vast majority of so-called generator quiet boxes are designed improperly and don't allow for the separation of cool intake air, hot "cooling" air that needs to be exhausted, and hot exhaust gases. I built one for my Honda EU2000i that's based on a proper design by forum member
jauguston, built for his EU1000i. I didn't want to invest a lot of $$$ in this project and could have improved on the performance had I been willing to spend more, nonetheless the results were quite impressive. Pics and videos are near the end of my gallery,
starting here.