Since the 986 and the 996 have a great deal of commonality, I have wanted to try fitting 996T liners for a while now (I would image GT2's would be far to expensive). I believe the T part number is: 99650412400 (one side). Will they fit??
I believe the GT2 bumper actually sealed the bottom rad exhausts and the 996T had open bottom rad vents, while both had liner vents. So you would likely have still make bottom panels to seal the bottom (provided cooling is adequate).
Here are some visuals:
997:
Maybe those that are not in the frozen north can give this a try after a trip to the local dismantler!