You can try brass as well. The copper might have been too thin if it stuck. The copper and brass release the heat very fast and shouldn't stick but I usually use a block of brass and/or copper. I weld a lot at work and sometimes here at home so I have learned a lot and different tricks to fill in big gaps. A mig is the easiest and best way to fill in gaps. Maybe next time try turning down the heat. Whatever works for you is the best way to do it. Just my two cents.
For an even smoother look, remove the turn signal bezels and make a larger lense that will fit flush with the bumper. Here is what it looks like...
Got a little side tracked on the project. I bought the fiberglass bumper fillers for GBoby. They fit perfect. So perfect I'm ordering another set. I will get some pictures up soon.
Not sure if you've checked your local laws but it's illegal to blacken out your lights in some states. It's a 2 point ticket here in Michigan.