gouge or scratch
There\'s a medium size x-acto kit that comes with some gouge blades (like chcisles with a V shape edge)- they are designed for removing material from a surface t create a depresion or sunken line. You could use one of those, but the depth would be hard to control and you probably would get a croked line. The same kit comes with a tiny plane- if you could mount the gouge in the plane, it would control the depth and make using a guide very easy.
A similar concept (in reverse) is to scratch away the surface by dragging something hard and sharp across it. I think this is the approach I would take- it allows multiple, controlled passes. All you need is a ruller and a large sewing needle, or something similar. Best would be the corner of a very thin, hard piece of metal, or maybe glass (cut or broken). You may have to clean up after with a little sandpaper, but the raised lip (if any) will be smaller than an exacto might leave.
I think in many cases, multiple layers would be easier, and look better. You could even cut the outer plates so the edges were beveled.