Are you thinking about a permanent enclosure for a portable generator, a permanent enclosure for a generator set in place, a temporary enclosure for a portable generator,....

I guess a Permanent enclosure for a portable generator. I want to be able to store the generator all year round in this enclosure and have it ready to fire up in case of an emergency inside this enclosure protected from the outside elements.

What you seem to be seeking seems not possible. If you want it protected by the elements during non-use, you want an enclosure that won't allow exhaust to escape when it is running.

I would look into one of those plastic mini-sheds, and locate that next to a pad of some sort--perhaps using flagstone or pavers. Then when you need the generator, roll it out of the little shed onto the pad and fire it up.

Here is a link that might be useful: rubbermaid shed

The trouble is that it would have to be very large in order to be sure you cooled properly. Before it got large enough to be cool, you could then walk into it and then it becomes an asphyxiation hazard. What you need to do is build something with completely removable sides.