Hughes,
I run a good oiled foam filter. On every bike I always ductape the airbox gaps to cover gaps between the rear fender and frame tubes. Makes quite a difference in the amount of water that can get in. This may be a holdover from the old days when you HAD to do it to keep your bike alive - but it still works. With the 250 rear fender there were also some extra gaps at the seat edge. I filled these with chunks closed cell foam. We get lots of nice mud in the spring and I have no problems with getting mud into the airbox. Any water in the box goes out the drain in the bottom. I cross a few seat high creeks with no issues (other than getting wet). Hope this helps. Cam.