These should work well:

http://www.crudproducts.com/

They are VERY close tolerance on my BMC running 23mm tires. I had to do a bit of re-engineering to get them to fit, and any bit of sand or grit makes an audible sound against the tires until it works itself free. On the other hand, they work well at keeping water off of me, the bike, and off of the guy behind me.