A thin sheet of copper.
Copper is a soft metal, not hard
enough to scratch glass. Keep a square of it next to the cooktop and use it between the glass and the cast iron. One added advantage, since copper is a better heat conductor than cast iron, it will distribute the heat on the bottom of the cast iron pan, making for less scorch spots.
Where to find it? Any place that deals in roofing supplies
should have rolls of copper flashing.