Im a cheapass so I epoxy'd two washers in place of the bottom screw holes, then used Automotive Goop to glue the back of it down. I used Goop exclusively as the tabs were broken when I bought the car 5 years ago, it held for about 2 years when the Texas heat did the glue in. I just added another dab of glue and it held fine. This time I added the washers, it should hold it in even better.
Buying a new one would be the best way, but personally I can think of a dozen things on the car i'd rather spend $120 on.