If you want it to look OEM CenterIsl outlined what you have to do very well, and it will cost quite a bit. I have only heard of two or three people that took the time to add the door speakers so they look OEM.
One member here just took his Boxster to a stereo place and had them cut a 6.5" hole in each door and install speakers. He spend around $3-400 for everything and used plain black mesh grills. It turned out really well, not OEM, but not some stupid looking install with chrome speakers.
Also, here is a good guide to disassembling the doors and some speaker information. Try a search on here and on RennTech for more info, there are a lot of threads on this.
http://images2.wikia.nocookie.net/boxster/images/b/b6/Boxster_speakers_update_article.pdf