[centerpunch]

I was also worried about screwing into the thin plastic panel, but it actually has cylindrical posts on the inside, centered behind the dimples.

You're actually not supposed to drill through the dimples, just use the screws they provide and they will thread securely into the posts hidden on the back side of the panel. (I did drill a smaller pilot hole anyway.)

Bottom line, the rear license plate mounting is more secure than it looks.