I have run K&N stage 1 intake on my Harley for 40,000 miles through every kind of weather. So completely open behind the chrome (open back) cover. Never had any intake issues, ever. Yes, a prefilter is nice, but really not necessary especially for what K&N charges for them. The oiled filter essentially does what oil does, shed water.
Getting water in the throttle bodies has never been a care or consideration.
I ordered everything today. Should have it for a nice weekend project.
Not looking forward to removing all that duct work!