Happy to help, but I'm a somewhat informed amateur at best when it comes to this stuff. There are a couple of *somewhat* helpful discussions at DYIMobileAudio.com that I've come across:
This one introduced using the Morel CAW 538s as a replacement speaker:
https://www.diymobileaudio.com/threads/turning-a-boxster-bose-enclosure-into-a-real-sub.135149/
This one discusses other possible aftermarket speakers beyond the Morels:
https://www.diymobileaudio.com/threads/please-help-design-a-sound-system-for-porsche-boxster.156035/