Fantastic Fan warranties the vent cover for life. Contact the company and they will ship you a replacement at no charge. I had to replace mine in 2008, it broke in the same place as yours JWSwift. The company mailed the replacement within 5 days, to where we were staying in Bisbee, AZ.
Here is a link to the contact information:
http://www.fantasticvent.com/contact_us/contact_us.html
You can find the model number on the fan body. It will be easier to see with the screen removed or by lifting the vent and peering in from the roof. My contact person at Fantastic Fan was Shelley Dunbar.
Let us know how you make out.
Dick