Covering the window is a good idea. Unlike a tow vehicle, a trailer follows right on the heels of another vehicle and is vulnerable to rocks thrown up by the tow vehicle's rear wheels.
Here is an idea -- using short self tapping stainless screws, attach C-channels to the front of the trailer above and below the window. Slide a piece of plywood into the channels. Rig up pins at each corner of the plywood so that the wood does not blow off.
At the campsite, remove the plywood rock guard.
Obviously, this will have to be carefully secured. The tolerances will have to be pretty precise.
Good luck!