Danno,
The cap I believe you are referring to is actually called the Aston Quick Release Cap. It is available in several different diameters. The one used on Cobras is 3 1/2 inches in diameter. It is actually bigger but they are talking about the size of the original fuel fill area when you open the cap up. This size also comes in different combinations of mounting plates so different diameter hoses can be used. Look at
www.europaspares.com and you will see a complete selection of caps.
Bill K