Any recommendations for a backup cam?
 

It will be connected to an aftermarket head unit.

I fitted one of these, cheap and works well enough, fitted it in the lip above the number plate (need to drill a small hole) and it's pretty much invisible unless you're looking for it...

https://www.banggood.com/135-Degree-Car-Rear-View-CMOS-Front-Back-View-Forward-Camera-Reverse-Backup-Parking-p-1040231.html

I added an AUTO-VOX Cam 6 from Amazon ($20). Works great during the day, have not really tested it at night yet (car still being built).

Recently on my Audi S5 I used a package from NavTV that consisted of front and rear cameras as well as the control module for integration to the existing MMI. Granted this was a professional install as I was looking to achieve OEM functionality. I would like to do the same to my Boxster, but as a purist, I don't want to replace the CDR.

You might want to check out NavTV...they have done some pretty cool stuff, and have a huge range of products (including a FLIR camera, which I seriously considered). If you do find that they have a solution for a 986, I would be interested in using them again.

EDIT: after posting, I looked at their site and saw that they have an LCD overlay/rear view mirror combo that would allow a camera input. I might be seeing a future purchase/solution for me. :)

I have used this one on a prior install and liked it:
Http://amzn.to/2nf441c

I also recently installed one of the rearview mirror units, with link to the video below.
Install of Dashcam Backup Camera Mirror - 987 BOXster