Fenders are nice to have when you need to ride in wet weather. Road gunk spraying you from below is a lot worse than rain pouring on you from above.
I wanted a fender for my 26" front wheel. Turns out a license plate works perfectly. It's the right length and width and can be bent to any shape. Plus it's highly reflective for night riding.
Unfortunately for us Yanks, U.S. license plates are too short and fat for the job, but European style plates seem to have the perfect dimensions (520mm x 110mm).
Luckily for me I had a souvenir plate from Germany that my sister gave me. I had no problem deeply offending her by destroying her gift. It says "BITBURG 05." Bitburg is a small German town whose claim to fame apparently is that Reagan caused some controversy by visiting in 1985.
If you are in Europe (or apparently certain former colonies), the first step will be easy for you. Go outside with a screwdriver and steal the license plate off the first car you see.*
I didn't need a rear fender because my rear rack already blocks any spray. You would probably need 1.5+ license plates to cover your rear tire.
* Disclaimer: don't do that!
Step 1: Materials
You will need:
-a long skinny European style license plate
-a piece of flat aluminum bar stock (I forget the exact dimensions but about 1/8" thick, 1/2" wide, and 30" long)
-2 u-bolts (the smallest size that will fit around the blades of your forks - mine had shocks and so may have been thicker than yours)
-shears that can cut through sheet metal
-a pop rivet gun and 1/8" rivets(or some small screws and nuts would work instead)
-pliers or a vice are useful for bending the metal