I was just going to post a thread on how I restomodded my grill. Anyway, here is the answer to your specific issue:
Use a aluminum wire screen to offer structural support and strength. I used plastic epoxy putty. The mesh allows you to work with a much thinner piece of plastic putty:
Work the epoxy putty into the mesh.
Push the stuff in from underneath so that the structure will be underneath and the top can be finished smoothly. You can cut the mesh larger so it flanks down the sides inside as well.
Work putty into the missing chunk of grill support from the top now:
Fill the shape of the whole piece up while imbedding the mesh from underneath with another layer of epoxy. Don't make it too thick or your retainer clip won't fit. Here is the piece partially sanded:
Completely sanded:
Drill the hole out as per the original, and you are good to go.
Sorry about the weird pics. Some are screen caps of a video I made demonstrating this. I didn't really get a good picture after paint, but here is the area after installation. None of the work shows from the outside obviously. Another option I thought about was embedding a washer with the epoxy and mesh. The idea is to give the screw something to hold on to without pulling the whole thing apart.
Here's my grill when mostly finished: