Making your own soundproof generator enclosure can be a fun and satisfying task.
Most materials can be found easily at your local hardware store or online.
Here is a great list of materials that can be used and some facts about them...
Mass Loaded Vinyl

• Great noise reducer
• Often used on the outside of a good soundproof shed
• Is tough and durable
• Can be layered
• Can be painted for a more decorative look
• When applied, gaps need to be caulked or metal tape used for effectiveness

Medium Density Fiberboard (MDF)

• Used for the walls of the enclosure
• Preferred over plywood because it passes less sound
• Some are moisture resistant
• Some types are fire retardant
• Flexible and can be shaped or curved
• Inexpensive
• Heavier than plywood
• Emits possible carcinogens when cut

Closed Cell Foam Matting

• Great for inner insulation
• An effective soundproof barrier by blocking and absorbing noise
• Water resistant unlike open cell foam
• Known to be mold resistant
• Usually fire retardant
• Lightweight
• More durable and stronger than open cell foam
• More dense and costs more than open cells
• Some expensive models are a "peel and stick" design

Fiberglass Insulation

• Inexpensive, compared to closed cell mats
• Moisture resistant
• Non-combustible, non-flammable
• Does not easily deteriorate
• Effectively controls temperature and sound
• Needs to be tightly packed for the most optimum sound barrier
• Skin irritant

Acoustical Foam or Tile

• Fire resistant
• Effectively absorbs sound
• Some absorb water

Soundproof Paint

• Blocks moisture
• Reduces noise and vibrations
• Non-toxic
• Requires several coats to be effective
• Flame retardant
• Maybe hard to find

Generator noise can be a big problem, especially in quiet neighborhoods or campgrounds.
These soundproofing materials would definitely be a great asset in reducing noise pollution.