Hi,
I was going down that lane myself some time ago and did some research
there are very good instructions at
http://boxsters.addr.com/hacks/harness.htm
I BELIEVE (it's not confirmed) that the Brey-Krause parts which you will need are
R-9001 and
R-9003
HOWEVER (and that's where I stopped) when I did driver education in preparation for DE/TT events I was told that running 5 or 6 point harnesses with a stock Boxster seat is actually quite dangerous. The stock seat is kind of shaped like an "A" which means that the shoulder harnesses will have the tendency to slip off your shoulders. And I was told that they will do that at the worst moments
This means that you will be safer with your stock 3-point harness.
I live in CA and if I want to do TT in my PCA region I need to buy racing seats (the one's with the holes in the back for the shoulder harnesses) to install 5/6 points
Better check that with your chapter before you spend a lot of money on something they won't allow you to use.
Let me know how it goes
Christopher