We have an '00 Boxster S, with a couple of simple mods I've installed.
The SmartTop relay is a nice addition, that works well. I know some use the relay bypass wire and pulling up the handbrake one clik as a cheaper alternative. But the SmartTop is easy to install and reliable. I really like being able to press the top button once, while the car is under 30mph, and the top automatically goes full up or full down.
The "Red Clip" is so inexpensive, I can't imagine trying to make my own. I suppose its possible. Some people also paint them black so they're not so obvious/visible when used.
As far as a GPS, I'm still a fan of the portable units. We have several cars, and I like being able to use by Garmin Quest2 in any one of them.
Don't know if you've seen Porsche Pete's Boxster Board yet, but that's where you'll find a ton of Boxster friendly folks and information.
http://www.ppbb.com/board/986board.htm
I've also done the garage door opener and OBC switch hacks to our car. Instructions are available in the PPBB's Archives.