Using the hub to center the rotor is the best way, since in the future you might get rotors with that inner hat diameter different than what these are.
Since you state there's a 0.100" hub diameter difference (3.050 vs 3.150), you could get a pair of stainless spacers made which are 0.050 thick. Make them a slight interference fit on the hub and a slip fit in the rotor. Obviously only as wide as the rotor hat thickness, so that they don't interfere with the wheel hub hole.
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