I wound up buying this pack of four 1/4" cork tiles at Lowe's...

http://www.lowes.com/ProductDisplay?productId=4747227

I'll test it at 275 F with my soldering iron first.

Should be fine: nice and light, and generally flame retardant. We use a set of cork pot/skillet holders when we have a buffet, and they seem to work fine... no scorch marks.