I'm using the Billet Specialties 3rd brake light license plate. It's a little higher than the brake lights, but Cobra's are low to the ground and drivers are looking at your car and not your brake lights.
http://www.billetspecialties.com/pro...cid=6&scid=187
If you do the third brake light on the rollbar, make it removable. Wire up a "plug" behind the driver's seat and zip tie a light to the rollbar. Then take if off when you meet with other cobras ....