I'd kill for a 2dr TCR in the states...
On each side of the stock piece, on the edges closest to the rear wheels, there are two screws that thread into a plastic insert (part # GM807509). I couldn't figure out a way to reuse those inserts with the TCR version. The mounting holes lined up perfectly, but without the inserts there was nothing for the original screws to thread into.
Either there is a TCR version of that insert or the bumper itself is a little different. I ended up just using some nuts and bolts I had with a dab of loctite. There are zero fitment issues, rattles, or anything else. I just think there is probably a more correct way to secure the corners.
Here is the edge of the stock piece:
And here is the insert: