What can I use on my roof to cover unused/never used attic fan

A/C installer came for annual inspection and noticed light coming through in attic from what appears to be an obvious blowing away of the attic fan cover that was on our roof just the year before. [there was no light coming through then] This fan has never ever been used and he noted the wires were even cut so it wouldn't work anyway. What can we use to cover the space that this missing cover has left so we can prevent any further water coming in. He said it was probably a round dome that blew away in a storm or something, but might not be available anymore and to check to see if we could find a large square cover/dome. Does anyone know where we can get this item or what else we can do? Thank you.

 

You can buy replacement covers from Broan/Nutone. Not sure it would be the right size or screw pattern. Broan-NuTone

 

The best way would be to install a new one and wire it up.
What other vents are on the roof?
That fan exhost the hot moist air trapped in the attic.