How can I attach/seal around an exhaust fan duct at the roof?
A proper roof vent would mate to your ducting. The seal will be between the duct and the vent via an integrated collar. You don't seal between the vent and the roof (at least for this purpose--rainwater is another issue). The vent flange should cover those corner gaps without issue.

I'm not sure you really need 8", though. Most such roof vents are going to be 6". I can't see a typical kitchen vent needing that much capacity. I'd get a vent with a 6" collar and a reducer. It'll be dandy.