center brake light

OK I put one in our first 67 notchback fish many years ago. Used a GM type fixture and cut its base into 3 pieces modifying it to hug the rubber flange close as possible to the glass. This is a must to prevent glare on the glass obstructing view. Very same done in the red 67 notch I drive today. There is a easier better way.
Slantsixdan has posted links to retail fixtures that adjust to fit various applications.
Now about the wiring... When adding a 3rd brake lamp to vehicles that use a single bulb ( 1157 ) for both brake and turn, a logic module in the wiring prevents the brake lamp from blinking with the turn signals. Those logic modules never lived more that a couple of years for me. I found it much better to just tap the white wire between brake lamp switch and steering column, and route a white wire the full length of the car along the original harness and into the added lamp. No logic module required.
Also note, Where vehicle safety inspections are required even this added lamp must work to pass inspection.