I could be wrong but, I think we're about to open a can of worms here.
Even though it's just a hood fan, it does have the potential to catch fire.
Over time, the exhaust will build up flammable grease.
Given that, do you need double walled stainless steel exhaust?
Are there any fire codes to be considered regarding this exhaust?
You can apply a trim ring around the pipe and give the opening a finished look.