3M has quite a few products that do work well. Here is a fix that I copied from here a while back.

Start with 100% pure Acetone, buy a gallon.
Pour an ounce or so intp a glass jar (small open mouth jar). Take a scrap grill and using something sharp, scrape off peelings (like pencil shavings size) into the jar of acetone. Put allot of shavings in, the acetone will gradually melt the plastic into a goo. Add enough plastic until the goo reaches toothpaste consistancy. Once you have a good batch melted put the lid on the jar tightly and set it aside. Now, prep your grill as normal, V out cracks on both sides, etc. Use clamps near the joint to alightn the two sides flush and parralell. Smear on some plastic goo, leave it crowned up over the crack like a regular weld looks, let it cure overnight. Sand to flush the next day, fill any pinholes with JB Weld. The crack will practically dissapear at this point, paint as normal