My 'rear fender' is a rectangle of coroplast cut from an election sign. It's maybe 3-4in wide, and double the length of my rear rack, and it lives folded in half on top of the rack, with the front two corners ziptied on. When it rains, I fold it out. Works pretty good, good enough for SoCal where it 'never' rains (right now is the first rainy week since, I dunno, May?). But if I ever move somewhere with real weather, I aspire to get some cool wooden fenders