How to get the nuts/screws? out

Those are factory anti-theft screws so Juvenile Delinquents don't steal your flip-open gas cap. I know some guys back them out from inside the trunk using Vise-grips on the threaded shanks but......

I bought a socket to fit them from McMaster-Carr about 8 years ago. IIRC, there were 2 sizes listed and I measured the head diameter of the screw and matched it up.