I have one of these that had similar problem, if used properly I don't think there's much heat involved, I used some very thick and very stiff card, and to be honest I think its the same or very similar stuff that was originally used, its red in colour and about the same thickness, it has a waxy feel to it, and can be cut with a fret / junior hacksaw, its the stuff used to make clamp bands on electrical wires like in the old type 3 pin plug, I think I still have some kicking about, you are welcome to have some, for the price of postage if I can find it, if your interested let me know and Ill look………
HTH
Shaun