I've used the material available at automotive upholstery shops. Its a black cardboard product approx .06 thick.
I've also used plastic "cardboard". It looks like corrugated cardboard and is commonly used to make shipping containers. It is also used to make small signs like those seen around town near election day.
Probably can't beat it for fire retardancy either. Ouch it's expensive though...I used Nomex felt, 1/2" thick, as the backing to attach the trim fabric to. It's stiff enough to support itself and works well as insulation too.