Number one: Get a quieter generator.
Number two: spend a lot of money and time trying to get your current unit quieter.
First realize you can NOT in any way, impair the air flow around, across and through the generator. Doing so could have an adverse impact on it's life.
You can use a sound insulation material around the box, this will reduce the sound transmitted by the generator directing into the frame of the. But (SWAG) the best this will do is reduce noise by 3dB (or half the energy of the noise NOT half as loud).
You can also look at an improved muffler to reduce the noise, but as with air flow, care here is required.
Personally (I worked in sound/noise/etc for many years) I would get a quieter generator.