I had the same exact thing with the same speakers, took a few pictures of how i did it but took most after i build it so pictures may not help, but i posted them at
http://www.danieldodd.us/Boat%20Projects/Tower%20Speakers/index.htm Basically i bought a piece of 22ga steel from Lowes and gut out a ring to mount to the can (make sure you paint). I cut them out to the size where the speaker grill covers them up. They don't look bad at all, you cant see the bracket i made, and sound pretty good.