What can I safely put between my cast iron skillet and induction glass top stove to prevent scratches?
You could use a thin disc of copper...

Copper has a much higher melting point than does aluminum (1,984ºF or 1,084ºC vs 1,221ºF or 661ºC), is not ferromagnetic, and a thin (.025" or less) sheet should not have a profound effect on the magnetic field induced in the iron skillet. 

I have not tried this method, but it is scientifically plausible as well as cost effective at about $25. 

The copper surface could be sanded smooth and could be used in a similar way as a heat diffuser. It should get no hotter than the bottom of the cast iron skillet, since the induction 'burner' should not induce any magnetic field in the copper sheet itself. The only heating of the copper would be from conduction or radiation from the cast iron pan.