I just Googled "Metra Boxster" and found a number of vendors that sell Metra part number 99-9605B, the same kit I used when I installed a double-din in my '02 S. These include main line retailers such as Amazon, Walmart, and Best Buy as well as specialists like Crutchfiled.
The kit includes a frame to mount the HVAC control in the lower console. My car's HVAC control was already in that position, so I didn't need to use that part, but it sounds like that's exactly what you need.
The kit also includes a frame to mount the double-din head unit in the dash. I did need to modify this frame a bit to properly fit my head unit but it's a well made part that that looks like it belongs. I'm well pleased with mine.
more on my double-din install here -
https://rx19jones.wixsite.com/projectboxster/projectboxster-3
Be sure to post a photo of your install when you're done!
dj
