"However, the two halves of the clamp are not completely closed up. There is a slight space between the two halves at the outer edges."

Is there a space between the two halves where the clamping bolt is? My concern is that the bolt may not be exerting force on the seat rails but rather is just clamping the halves together. Is it possible that shimming the rails would cause them to be clamped tightly? Perhaps your seat rails are somewhat undersized, as you are wondering.