I made a custom paper cutter for a student competition where they need uniform strips of paper and they need a lot of them quickly. I designed this in 123D Design using net dimensions for the razor blade so they would press into the plastic. They are exactly 5/16” apart verified with a caliper. This will work well if you set it on some foam insulation and cut along the paper with a ruler.
The razor blades were pressed in from the bottom, creating a tight fit so they don’t fall out.