You could make your own out of Lexan. We used to make them in our metal shop class using a plastic strip heater. All you have to do is heat the plastic and make a single lap joint on top and bottom. Then slide it over the license plate. Then put a dab of silicone caulk at end of each lap joint to hold Lexan in place.
Use Lexan plastic as it is more durable and more sun resistant than plexiglass.
Here's an example of other items we used to make these frames for. This poster measures 12 x 14. Most shops that make plastic items or sell plastics could make you a plate cover while you wait. Very easy to do.
What's neat about is that it looks invisible and there's no "frame" to hide the poster.
Plastic strip heater info here:
https://www.google.com/search?q=plas...hrome&ie=UTF-8