Passenger back rest for a CB750F

Have a 1975 CB750F. The wife won't ride it unless I put a back rest on it. I have searched e-Bay, etc., and have found sissy bar type back rests that say they fit. The 750F's seat opens up all the way back to the stop light, so I wonder if the sissy bar type seat has enough gap for the seat to clear. By the way, anyone out there have a back rest that will fit for a reasonable price ?