Welcome, KommMaster!I know it's not the quick answer you're looking for, but have you searched the archives? There's gobs of info and advice and pics and video links for silencers, filters and seating for the C5.
I like the air box that St. Beardface posted you can source the parts from hardware and auto parts stores. Have not tested it myself but I like it!! http://www.enfieldmotorcycles.com/forum/index.php/topic,12468.0.html