Open cover not enough
The previous owner of my boat had one in the lazerate without vents plumbed to the outside. I can only surmise that he had the lids to this compartment propped open when the generator was running, since he's still alive.
Nevertheless, the inside of this compartment and down in the fuel/battery hold the walls were covered in a black greasy soot that was a bear to clean.
I've got a small Honda generator, and often thought about putting it in this compartment and plumbing the exhaust to the outside. However, there's already way too much weight in the back of this boat. So, when it's not in use I keep it stowed under the V-berth, just in front the access door that leads to the under the berth. It's out of the way, and when I need it, I just lug it out, place it atop the lazerate and run it there.