My trike has a fender on the rear wheel, and it's there all the time. If I didn't have that, and in your situation, I might consider zip-tying a piece of coroplast in place. That might require a rear rack or something so that there was something to zip-tie it TO, though. BTW, I did something like this with my old V-Rex: I put a piece of coroplast behind the mesh of the seat back. It at least kept my back clean.