I like black ones. The reasonably heavy duty plastic of 10-litre oil containers you will find in department stores work quite well. You can create your own shapes, it curves quite well to conform with the shape of the fenders and you can make them reasonably long.
However, don't go too stiff. Based on my experience, a good flexible mudflap means you won't break anything should you get it caught on another external object, and it won't break your fender should it get caught up between the fender and the wheel. It also enables you to rest the fork dropouts on the ground when removing the front wheel.
YMMV a little.
Last edited by Rowan; 09-15-14 at 05:26 PM.