A cone filter will fit right over and clamp to the neck of the AFM. These days, though, ITG foam filters are better than K&N paper and screen filters. They flow better, last longer and are easier to clean. Our M2 has an old K&N that's due for replacement and I've been much happier with the ITG that is in our E28 (about 2 years old, no sign of wear).
Here's ITG's cone chart:
http://www.itgairfilters.com/airfilters.asp?section=maxogen
US suppliers for ITG are a little difficult to find, but google will turn up several.