When I couldn't find a prefilter for my tractor's air cleaner, I made one from a universal foam air conditioner filter (the kind you can get at Walmart for window air conditioners). I just wrapped it around, cut it to size, and hot-glued the cut ends together. It slips on and off pretty easy and has worked good for over 4 years now. Hint: To oil the prefilter, put it in a large ziploc bag and add about an ounce of oil. Seal the bag and then work the oil into the filter. No mess, no oily hands.
