Just a thought Fireball, over here you see a lot of bikes with a milk crate on the rack for storage and transport. I always thought that would be a great way to hide an engine and get some sound proofing. The sound waves bouncing off the ground would still echo but the top being open would direct a lot of noise up and away. Would need some sort of sound absorbing walls and at least some of the bottom cut away. milk crate+bicycle - Google Search
Just a rambling thought.