Righto, we're talking cross-purposes.
The panels you are referring to are what I'd called the taillight surrounds - my term for them, anyway. Yes they are plastic. The surrounds differ between 2+2 and 2-seater, the 2+2 surround being slightly bigger.
Whereas I was starting with the basics, asking whether the metal (structural) panel, of the body shell, is all 100% straight and not accident/rust damaged/repaired.
It might be hard to photograph but harder still to diagnose without something to look at.