I have my Honda eu3000is mounted on the Stromberg Carlson toungue mounted cargo tray that others have linked above. It works great if you can stand the extra tongue weight up there. The only issue I had is that I had to delete the hard plastic cover that was on my twin 30lb bottles. Since the cover would no longer be removable with the platform installed.
Other than that I have had no issues at all. No problems with noise, vibration or fumes. Maybe others have had those problems but they have not been any issue with me. When the generator is running I can't even tell it from inside. The A/C noise completely covers any noise made by the generator.
For me it is a great solution. I tow with a Nissan Armada and don't even have an issue with opening the tailgate. It clears by about 6 inches. I just run the TT cord up and plug it into the generator and away she goes. I do have the generator locked to the platform with two padlocks and two cable locks. Keeps out the casual thief but if someone really wants it, has the right tools and has enough time they can get it but I don't really worry about it at all.
Also, I keep the generator covered with a Honda cover and put an outboard motor cover I got from Bass Pro on top of the Honda cover. Put a small cargo net over that with some bungee cords to keep things from flapping in the wind. Generator has never evne gotten damp. If I had to run it while raining, I'd have to come up with some kind of "tent" over the top but that hasn't happened yet.