I'm in the process of test fitting everything on a Misha GT2 decklid and I'm trying to figure out how to mount the 3rd brake light.

Attached is a picture of what I'm looking at. This is the end of the brake light that has the plastic key you turn to release or lock the 3rd brake light housing. On the decklid, it is just filled in with fiberglass. For those of you who have the same decklid, what did you end up doing with this to allow mounting of the brake light?

I was thinking about removing the white plastic locking piece and just screwing this end into the fiberglass. However then the red cover piece for this corner will not have something to snap into. Any ideas?