I agree with AStitchInTime. I buy all my template plastic from JAF -- both the mylar (heat resistant) and the regular. If you really, really want to use their template plastic, you would need to glue together several layers with e6000 glue because otherwise it is super flimsy & flexible. You would get more support from a cut up cereal box than from template plastic.
Or you could maybe just glue the template plastic onto something thicker like cardboard, foam or ShapeFlex.