Porsche sells such a kit and the part number is 000 044 200 11.
It is a bit interesting why Porsche came out with such an adapter.
When I started in 1999 the only Boxster was the 2.5 and Porsche did not have a sport exhaust option for the 2.5.
The sport exhaust started as a factory option for model year 2000 when the 2.7 and 3.2 came out. Then Porsche started to sell the sport exhaust in a retrofit kit form, so that you could have this long after your 2000 or newer car was built. As a factory option or in kit form the sport exhaust was and is expensive.
But the 2.7/3.2 sport exhaust muffler would not mount on a 2.5. So Porsche came out with an adapter to mount it to a 2.5. This adapter bolts to the transmission and muffler.
Then the 2.5 owners wanted a dual tip muffler, and the cheapest way to go was to get a 3.2 standard muffler. To mount the 3.2 muffler on a 2.5 clever owners figured the easy way was to use the sport exhaust adapter.
Need to write a book one of these days before I forget this stuff....
