You can stack adaptors or visit any half way decent machine shop.
Stacking adaptors might be the way to go because the amount of thread would be less, and therefore more likely to fail if the pad got into a situation where damage to the car was worse than the failure of the shaft. It might strip the threads on a regular backing plate, since it wouldn't go in as far as normal. Still, it would take a lot of torque to make a stack of those things fail.
Robert