I presume it's for your actionbent-type hardshell? Get a piece of open-mesh foam from Poweroncycling
http://poweroncycling.com/itemdetails.cfm?id=558
Water just goes through it. If it gets rained on, you shake it out. If you have one of those seats with a slot down the middle, and you're having problems with wheel spray, you can put a strip of for-sale sign under the foam to block it off.