TWB - I mounted my tweeters on the "sails" on my doors. The sails are the plastic cover that covers the mounting screws for the side view mirrors. I'll post pics in a bit.
bgrab - I just used the stock plastic speaker mounts. You'll need a torx driver to get the stock speaker off the mount though. Once I got the speaker out, I cut off the plastic shield on the back of the mount so I could lay it on the MDF and trace it. If you use the front of the mount to trace it your holes won't line up because one side is thicker than the other.