Sorry, do not own an '09 Boxster so cannot tell you exactly. However, the guide should be on back of fuse cover panel and or in your manual. There will be more than a few empty blanks to choose from which are keyed w/ ignition (a couple always on too). Look for blanks on fuse guide which should be keyed and are not on your vehicle... or simply test w/ a multimeter and your ignition key
Then, either use a "fuse tap" or a simple a .25" male spade terminal on the line side (w/ an inline fuse of course) to your accessory. A ground point is just adjacent to the fuse panel as well (there was in my 997.1 at least and imagine there would be as well in other 9x7 variants). You'll have to remove the surrounding carpeted panel but as I recall it's only a screw hidden behind the fuse panel access cover and maybe a clip or two.
Good luck
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