In all unseriousness, have you looked into the 3M coatings you can apply to the front end? I'm in Arizona, caught my first paint seeking missile rock on the ride home from the dealership when she was just a wee lil 300. I've never known this car without a rock scar, so I take pride in the starfield continually erupting under my black beauty's lower lip. My neighbor has a more expensive, yet less thrilling, vehicle that he had a 3M coating put on to stave off the rocks. Afterall, since he owns a lesser, yet more expensive!, vehicle than I, he will need to worry about things such as resale...when he decides to buy his own 300C and ditch the krautmobile.
He claims it works like a charm. I then asked him if the added weight from the 3M coating was perhaps the reason behind his performance sedan's performance numbers being so far less than those of my own cheapo Americano sedan. No comment.
A plastic backstop mounted on the front of a 300? Please don't do it.