I once made a rear fender using wire coat hangers and electrical tape: I took a couple hangers, unbent them so they were straight, then coiled one end of each around my seat tube about halfway down. I then ran each one up to each side of the top of the brake bridge and looped it around there to secure it, then ran them straight out behind the seat tube, over the wheel, and brought them together at the end, about two feet behind the seat tube. In essence, this makes a big outline of a rear fender that is secured to the frame. Then, you wrap electrical tape around it over and over again till the whole fender is covered. Et voila! Rear fender. (I wish I had a photo, because I think my description isn't so good. So tell me if this makes sense or not!)