I'm wondering if a Silpat silicone cookie sheet liner (Williams Sonoma) would work. I've used them in cooking and have always been amazed at their temperature range. I don't know if it would be flat enough but it's got a slight grippy feel to it and is designed so that you can peel it off the cookies in the same way that BuildTak would work. The quarter-sheet version is 11-3/4" x 8-1/4", for what it's worth. I have a feeling that the silicone would do great for this.