Those will not fit without significant modifications. If you compare your door panels to a Boxster with door speakers you will see what I mean. Your door panels were not designed to have speakers. I have a 99' Boxster w/o door speakers too. I also looked into a custom setup for the door speakers but felt it would cost too much.
You are probably better off upgrading the dash speakers and/or getting some put behind the seats:
http://www.iq.dynip.com/~gary/hacks/speaker/speaker.html
http://www.gagme.com/greg/Boxster/hacks/rearspeakers.php
Good Luck,
Larez