LOTS of engineering goes into NVH or Noise, Vibration and Harshness, in the automotive world. For these reasons, you barely hear a modern car at idle or when coasting down the road.
When you stand on the right pedal, some sound comes from the intake, some from the exhaust and some from mechanical actions, i.e. pistons, valvetrain, etc. New diesel engines (pickup trucks) have received HUGE improvements in mechanical noise reduction over the last 5-7 years, so much so, that they now sound like gasoline cars at idle.
A cheap or off name brand generator will see NONE of this NVH engineering. So, while you may be able to add a better exhaust or install some generator box liner from a company like Cascade Audio, you will not be able to do anything about the noise that the engine makes.
There is a reason that Honda generators are the quietest and most reliable on the market: Honda dumps millions and millions of dollars into making them that way.
Honda EU2200 costs about $1000
Cong Shen 2000W generator costs $400.
Why do you think that is??
If a 3500W genset is not enough, I would suggest two 2000W Honda or Yamaha generators, thereby offering flexibility, redundancy, and most importantly quiet.
As you probably know, used Honda generators hold their value very well, but can be picked up for $600 - $800 with some shopping. To quote an old commercial, "Buy it once, buy it for life."
You will be time and money ahead making a long term investment, instead of trying to make a cheap, loud generator, quiet.
If you insist, a large volume muffler will absorb the most sound, probably not something that you want on your generator and an in-your-face example of why most small engines, i.e. weed trimmers, chainsaws, etc., are so damn loud.