I recently purchased a 2017 RDX and was surprised to find large holes in the rear hatch behind the license plate and only two upper clip nuts for the license plate.
I bought two 6mm clip nuts and modified them to fit over the two large lower molded bumps. I had to drill out the bumps with clearance holes for the 6mm machine screws.
A thick plastic bag cut approximately the size of the license plate was then attached using packing tape. It’s not perfect but I believe better than nothing in keeping dirt, salt and other things out of the hatch. I added photos but they appear to be very large. I hope once this is posted the photos will get smaller.