The first "C" we bought came without a generator (or roof AC) and also was not set up for one. Instead of a generator hanging on a metal frame with insulation and sheet metal around it, there was a nice big sealed wooden compartment for outside storage.
A Kohler 4.0 fit perfect in the compartment, so instead of doing like the factory genset install I left the box.
I lined the box with several layers of asbestos paper and then cut and folded a piece of sheet aluminum to make a 5 sided box to line the compartment with. Screwed it to the walls and sealed the joints with hi temp silicone. Made a template and cut the hole thru the bottom for exhaust and air and added some vents to the door.
It was quieter outside than factory (my parents had one) and we could hardly hear it inside. Wood is a great insulator. I think it also helped to deaden the vibration.
I know those Hondas are pretty quiet already, but how about if you put it in a metal lined wooden box ? And paint it with a fire retardent paint.
" I hear a cat purring"