All right, guys, it's DOUJIN TIME.
Here's "Butterflies", which, in my opinion, is the best mature K-On! doujin out there. (I like it just for the MioxRitsu chapter, but hey. I suposse they're all good.)
click the picture to go straight to the download link, or here to read a summary and whatnot :3
http://myanimemangacollections.blogspot.com/2010/04/butterflies-k-on-doujin.html