One problem with most commercial fenders is that they don't extend down far enough to block all the water getting kicked up by the tires. Of course, if the rigid fender extends too far down, it can catch on things or interfere with loading the bike onto some car roof racks. So a good solution is a flexible mud flap. Here in the Seattle area you'll see many creative mud flap solutions including plastic cut from milk jugs or water bottles, hunks of coroplast or leather, sections of rubber stair tread and all kinds of other creative solutions.
A section of an old bike tire can make a very effective mudflap. The tire beads can be cut and removed with tin snips or some stout scissors. Zip ties running through a couple of small holes drilled in the fender hold the flap in place and the rubber is stiff enough to hold its shape and channel the water away from you and your riding companions.