I'm confused, don't you apply pressure to your ruler when you are using it anyways? I always have to put my weight on the ruler or I can't cut straight.
Im guessing the warping is because you're using pegs that angle upwards a bit. You may be able to finger ones that are more of a right angle. Different retain displays use different pegs. Try reversing the way they hang (hang back side out) and see if that helps.