Low-Pro Products makes several great rail mounted accessories like optic and light mounts. One of their newest is their Offset Flashlight Mount Surefire X-Series. As you might have guessed by the name, it is designed to mount a Surefire X-Series light, or other rail mounted lights like the INFORCE WML, offset to the rail on which you place the mount.
Their mounts do a great job of keeping the light close to the rail and have a simple and robust, set-screw style mounting interface. The mount interface is also aggressively skeletonized to strike a balance between durability and lightweight.
Check out Low-Pro Products from AMS Machine.