I agree with your commentors that suggested blocking the panels...get the straightest one, do that first, pinning to SOMETHING with a grid... ironing board cover, gingham, whatever...now spray it with spray starch, heavily.....let dry and then unpin....did it stay straight? okay, then you have your measurement...now block all the other panels to that measurement, starching well.... if it did NOT stay the shape you blocked it, you are probably stretching to much one way or the other, re-wet and try again, a bit smaller.....starch is your friend whenever fussy cutting anything....