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.
I will second the use of cast iron on glass top stoves. We use small skillets to large heavy (2-handers) on ours with no problem. Of course out glass top is not new and was already scratched up when we started using CI
I have used my cast iron pans on my glass stove top. Mostly they get used on the gas cook top but the gas doesn't do LOW heat very well so sometimes I move over to the electric glass stove top.
One big thing with glass tops is that FLAT is important, or so they say. Most of my cast iron is flat on the bottom. They work well on the glass.
My most commonly used iron fry pan (Lodge) has a raised ridge around the perimeter of the bottom. On an older electric stove or a gas stove the ridge helps to keep it centered, and perhaps helps to keep heat from escaping out the sides. On flat glass it would prevent contact between the pan and the stove. No flat glass cooking for that one.
Some iron pans get warped so they are no longer flat as they were new. That also doesn't work on a flat glass either. Aluminum pans also get warped sometimes.
Be careful not to drop iron on glass. My step daughter lost her glass cook top when something fell out of an upper cupboard onto the glass and it shattered.
I don't know about scratching the glass. We haven't had our glass stove all that long. I suppose it will eventually get worn and used looking.