I have two of these covers. One on a Maxair and one on a Fantastic. the cover comes with four brackets that have a pin. Remove the screws in the desired location, install the bracket and screws, set the cover over the vent so the pins protrude through the holes, slide the retaining clip thru the pins to keep the cover in place. Done.
I don't completely understand your issue. Can you post a photo?
https://youtu.be/3jaTvf8ybk4