UPDATE: Some people have mentioned in the comments that this approach may prevent proper intake. While I never had any issues, you may want to try a different placement to avoid covering so much of the grill.
After getting a ticket for driving without a front license plate in California I start looking at how to mount the license plate on my Fiat 500. Not only does the Fiat 500 not have a front mounting bracket, there isn’t really a good place to drill into the plastic to add one. For $5 I was able to get a basic license plate mounting kit.
What you need:
Using two large zip ties, I mounted the kit to the grill and screwed the license plate. You may want to make two 90 degree bends in the end of the zip tie. This will make feeding it back through the grill easier. The whole process took about 5 mins. I will find out in court tomorrow if this is an acceptable way to mount a license plate.