Hotwheel tracks, corrugated plastic, any kind of plastic sheet will work if you don't care about looks or doing some extra work. Or you could splurge and spent 15 dollars on a set of cheap full coverage fenders.

On the topic of fenders, have you guys ever seen some video following some messenger around new york? The best part is when it starts to rain. I laughed so hard when he shows the camera how resourceful messengers are by pulling an empty 2 liter bottle of sprite, a coathanger a pair of scissors and a roll of duct tape from his bag. A quick edit later and the the most ghetto ineffective looking rear fender I have ever seen is done he looks happier than a pig in sh*t. I guess functioning fenders weight too much so he just carries a bag of fender making supplies with him wheverever he goes and then spends 15 minutes making a crappy rear fender every time it rains.