As I recall boxster has a 1999 Bloxster. So, you need to stay with a cluster that has a white, blue and black plug one the back. So, a 2000 Boxster S cluster will work on your 1999.
If you want to keep your Boxster's mileage correct (or close to it) you can send the cluster to
ODO-PRO.COM - Odo-Pro.com - The Digital Odometer Professionals and for $90 they adjust it.
Here is a link to a nice 2000 S cluster on eBay from a quality seller.
00 Boxster S RWD Porsche 986 Silver GAUGE INSTRUMENT CLUSTER 98664111300 99,687 | eBay