Not sure about the screws, I'd have to look at my sled to figure it out.
But we got a good dumping here on Tues. so I rode my sled to work that night. On the way there the throttle position sensor kept kicking in and bringing me to a reving idle until it unstuck. Got to work, pulled it in the shop and discovered a butt-load of snow in the box. Above and below the filter.
Not wanting that experience again I found a thick "scotch brite" type scrubbing pad, the woven kind not the spongy ones, and sprayed it with a silicone based Demoisturant. Took the airbox off and layed it over the air intake and zip-tied it around the lip. Not one problem on the way home (except for blowing my belt at 100+mph), but that's a whole new animal there.
Any type of pre-filter material will work fine, this is just what I had available at the time.