I would suggest contacting customer support at Fantastic Vent. They have always been exceptionally helpful in my experience. You can find them on the web at:
I had two Fantastic Vents installed in my '64 Overlander at the 1999 WBCCI International Rally in Dayton, Ohio. They have been one of the best additions to my coach yet. The Fantastic Vent technician who installed my fans used one of the Airstream special trim rings around the front fan (in kitchen area); and used the stock (existing) Airstream trim around the fan in the bathroom area. My Minuet will be treated to a Fantastic Vent at the Lansing International Rally this summer.
I know that there have been several reports of Fantastic Vent providing the Airstream trim to fan owners when advised that an installer was unaware or did not use the correct Airstream trim. It would definitely be worth a try contacting the company.
Good luck with obtaining the needed trim!