Here's a thread i did for my bobber project:
http://suzukisavage.com/cgi-bin/YaBB.pl?num=1248802832/0.
it documents the how and why for a trailer fender i put on it. lots of words, but some good pics too.
here's a more recent iteration using the stock rear fender from an xs650:
the trailer fender cost about $15 plus paint and supplies and such. the xs650 fender was free from a friend who is building a hardtail. old chrome jap bike fenders are cheap and abundant at some shops and ebay. both these required a decent amount of cutting, drilling, and sometimes hammering
. they both need to sit up off of the tire a bit to keep from scraping over bumps, but the xs looks better because it has more of a curved shape.