Protect your plate? Why?
My first plate lasted 20 years and could have lasted longer if I didn't purchased a second car before selling the one it was on. I had to plate the second car while the first one was still plated.
Plate covers can actually be worst on the plate longevity as it traps moisture and whatever road salt that could seep-in. A plate without any cover or borders is likely to last as long as one who have some kind of cover, if not longer because the plate can dry-out and dirt can wash-off easier.
Now, we have those stupid "smoked" plate covers, those tinted ones, yellow, blue, red... Now that's stupid and makes the plate harder to read. I doubt many peoples with a clear cover will get pulled-over. If the cop see the plate, the camera will also so, no reason to stop a guy with a clear cover, even if theoretically, it will be illegal.
At that point, it will be the discussion as "does it really impair the visibility?" and whatever plate cover will be deemed not guilty, as long as it isn't blatant like, rendering the plate barely readable to the naked eye.
Just look at all the stupid mufflers laws: even if you can ear the "powerful motorist" (in his head...) coming from 1km away, the cops will likely loose the case in court because there's nothing allowing them to actually measure the sound level. All the noisy guy have to say is "I purchased it like that from the dealer" and have a statement from the dealer that the muffler wasn't tampered with.