Safe liner to cover donut spare area (after d. s. removal)

[nookandcrannycar]

I want to have the option of carrying my additional steelie and tire with me on a trip (especially through a rural state like Wyoming or Montana), and still be able to keep the package shelf in place and the bench seat back up and secured. I have to remove the lockable load floor (that covers the donut area) in order to do this, and also to maximize space. Leaving it with no liner doesn't generate any noise (at least so far...only tried this on surface streets). However, I'd still like to line this area to the edges of the car. 'Spilling' above the plane representing the removed load floor is fine as well. What type of material should I use? (safest re fire, temperature, etc, and also most pliable to allow for maximum utilization of space).