Heath, are you talking about the guard over the front window, or a guard for the lower body, which is where most of the really heavy rocks hit?
If you're talking about a window guard, not much is needed for several reasons:
1. Heavy rocks thrown from the tow vehicle's rear tires
can't be thrown that high.
2. The front window is plastic, not glass, and is quite tough and hard to break. It scratches more easily than glass, though.
My Compact Jr has an intact window rock guard that is made out of thin fiberglass, and that trailer has obviously been towed many miles, including over dirt and gravel roads. The fact that mine has no holes helps substantiate my first reason.
However, I have had a car windshield taken out by a rock thrown by an oncoming truck. But this has happened only once in my many decades of driving, so I wouldn't worry about such an infrequent event.
Keep it light
and keep it simple!