Well, I don't know if that spinner has good knurling on the backplate or not, and you probably don't want to hear this, but it sounds like you might need a different spinner that has a good grip to it. Either that or try drilling the backplate and prop for screws or pins to lock it in place as suggested earlier in this thread.

I'm going to buy a Tru-Turn spinner for mine. They cost a lot, but if it keeps me from having prop slippage, it's worth it.

Jim