I have repaired broken tabs on plastic grills by getting a scrap piece of the same type of plastic and grinding it into small bits with a rough file and then mixing the shavings into a paste using acetone. Put the paste on in between the two parts and make sure they are stable while it dries. Once it's cured you can sand it smooth and paint over it. It's a very durable repair if you do it right.

Scott Smith Harms on here gets the credit for the acetone trick.

Do you have the corner piece? If not you may have to make one out of plastic that is the same thickness.