Originally Posted by Condoluminum
That looks kinda Home Made... You may need to dust off aluminum riveting and plexiglass forming skills to repair that.. It doesn't look like anything Airstream ever shipped from the factory, but maybe someone else has seen something similar...
The picture is not that detailed, but it looks close enough to a normal center rock guard that's been whacked against something and bent/broken, and isn't flanked by the left and right curved guards. (maybe they've been swung out of the way, or just not installed on the rear? On the left side of the photo is something that looks like the attachment for one.)
That said, if you don't want to buy a new one, the center one is pretty straightforward I think. Flat rectangular piece of plexiglass in an aluminum frame.