I remember dealing with those, they were typically used on commercial roofs. I don't think that that type of insulation board is available anymore. I can tell you it is the nastiest stuff one can work with, especially if it's hot out.There used to be rigid fiberglass panels for insulation, but I haven't found them anywhere. Do they still exist. What sort of shop might have them?
There are various foam panels available; which kind is most heat resistant?