I made and installed a DIY mudflap, made out of an old shampoo bottle, yesterday. Instructions (in German, but you don't really need them anyway) can be found here. Of course, today it was raining, and therefore I can say: they do work. They look a little goofy because they're Fructis-green but finding a black shampoo bottle is not exactly easy. Function over form.
Pictures will follow shortly.