on the k&n website they have a section that lists all their "dry chargers" as I think they call them. in that section there are dimensional specs for each of their tens, if not hundreds of "dry/pre chargers", so all you need to do (assuming they dont just make a dry charger for your particular filter, sometimes they do), is just match your filter's dimension with a sock that will fit. i have one on my open air filter, and it has only ever gotten wet twice, and that is during a winter season on the central coast of california, no more than a mile or two from the shore most of the time. when is has gotten wet the filter wicked away most of the water, and while not saturated, (the material they make them out of pretty much never gets completely soaked through), they were significantly wet and show when you wash them that under heavy water they actually do soak through. despite this, its kept my filter dry 100% of the time and i've had no problem
from what I understand, the pre chargers are not as water resistant as the dry chargers, and are made to more keep the filter clean- a filter covering a filter, basically. you can get them from amazon.com, or just google the pre/dry charger's model # in google to find a bunch of places selling them.