Mine usually stay on with the edges taped. My oldest pinter has the cork saggin like an old car headliner but I have never bothered to replace it. Here is what I have used most recently. Although, I don’t ever go above 60C, maybe that is the problem. Before the cork the thing to use was cardboard, probably much more of a fire hazard but a lot of people used it. Maybe you could just get a bit of the small round insulation and put it in the gap between the bad and the carrier. That would make a nice air gap. Sorry I just totally realized a super hot bed is something I have not tried since torture testing the robos, and my lack of information might not be helping.