Thanks for checking in. I'm still at the same place I was in November. Standard Carrera RS inboard seatbelt receptacle mount and the stock rear seatbelt receptacle with the plastic sheath removed.
The two handed operation is a bit awkward, but works, for now.
If anything, I'd like the receptacle to be shorter, like the one on my Lotus Elise.
I may see if that one works too. It has a shorter overall length as the receptacle itself bolts to the mount. Unlike the Porsche rear receptacle that has a length of webbing and a separate mounting tab.
I may also reach out to Car Bone to see if they have anything that may work better.