This details a simple way to add a virtually free fender to your bike to keep mud/rain/blood off of your duds. This design uses recycled materials and the only cost is your time. The presented design uses a rear rack as the support, but you could easily use some other found items such as wire from a tomato cage or similar to provide structure for the fender. Face it though, your bike needs a rack so just get one, or view my instructable (coming some day) on making your own bike rack out of used automotive brake and fuel line and get to it.
Step 1: Materials
The materials for this are easy to find. You need a plastic gallon jug. I used a windshield washer fluid jug since it had vertical sides and was pretty thick. The circular indents and thin plastic on a normal milk jug make it less desirable, so keep an eye out for a thicker jug like those used for vinegar, windshield fluid, etc. Pretty much anything will work. You will also need a pair of strong scissors or a utility knife to cut the plastic.
Additionally you will need electrical tape or some zip ties or other method to secure the fender to the rear bike rack.
The figure below shows the kind of jug I used, along with a pattern that I used for the fender.