The rear one is useful for keeping the grime from spraying up off of your rear wheel, into the faces of fellow riders. Rubber long enough to get close to the ground. I use rubber which is sold in home stores as stair treads. Any fairly thick rubber or plastic should work, even plastic from milk jugs. I prefer to use rubber instead of plastic because the rubber doesn't rattle.
Plastic is lighter.
I found that flexible rubber is more durable then stiffer rubber. My favorite source of rubber is a stairtread cover. Two bolts and nuts or a zip-tie or two pop rivets. I prefer to use amber, but a mix of white and red works too. More bolts and some washers.