I've done two cafe seats. The first was on my 76 CB750F. I took the stock seat pan and built up a cafe "Hump" on the back of it out of fiberglass:
Since it was the stock seat pan, it still used the stock hinges.
For the second seat, I bought a seat off of ebay and built a base that would still use the stock hinges.
(after paint)If you look in my build thread, on page 14, I go over the process that I followed.
http://forums.sohc4.net/index.php?topic=22216.325