To re-direct the noise of a generator, I have four panels of plywood, 3x4', that are loosely tied together on the short ends with rope. After the generator is running, I set these panels on end around the generator, forming a kind of box.

This provides enough of a baffle to direct the noise essentially upward. There is enough airflow to keep everything cool, the plywood is cheap and lightweight (it's 1/4"), and the generator can be seen if needed.

This is similar to the technology the movie industry uses on location shoots. Rather than attempt to muffle all the sound, they redirect the sound upwards where it doesn't much matter.

I suppose that thin rigid insulation panels could also be used, but they are quite fragile.