Vent Cover for Fan-Tastic Fan?

[Sopper]

I recently purchased a Fan-Tastic Fan vent cover from Camping World to place over our fan to allow use in the rain. The instructions discuss removing existing vent fan screws and attaching the supplied brackets for proper installation. I have no existing / exposed screws and the vent, with back grate, do not look to fit well.

Is there a specific vent cover that I should be using? What has everyone else used to cover their fantastic fan vent?

Thanks in advance for any input / help you can give!