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There are two Shimano CL→IS adaptors, SM-RTAD05 for hubs with big axles and SM-RTAD10 for skinny axles. Same spline, just different lock ring tool interface to clear the big axle. The Service instructions for the 10 show it being optionally used with the bigger lock ring which comes with the through-axle hubs, so it doesn't seem like there's a good reason you couldn't use the small lockring with the nicer and considerably cheaper 05 adaptor. Other adaptors are available, e.g. Superstar. The other option is just get a centrelock rotor, which may be cheaper than an adaptor
Get the cheap Shimano wheel and a CL t0 6bolt adapter?http://www.merlincycles.com/shimano-6-bolt-rotor-to-centre-lock-hub-disc-adapter-45223.html