The screws that hold the white cover on are self tapping, your options if they’re stripped out are, using a slightly larger self tapping screw, or drilling new holes, either option works just fine.
As for as a cover for the ac I can’t help you, I had a bird nesting in mine recently, I find it just as easy removing the cover semi annually to clean, while i’m waxing, as opposed to getting a cover, you have to get up there either way.
Steve