I wouldn't worry about it. I made a kydex belly pan, with sides that zip tie on to enclose the engine. I cut a hole in the belly pan for the header to protrude at the very front and made a bulge out of some aluminum to keep the snow off the header. The edge of the kydex is probaly 1/2" away from the header right after it comes out of the engine and it hasn't melted or gone brittle. No issues with it cracking either when it is really cold out. Its pretty tough stuff.
