This tool holds a razor blade at a 45° angle to cut classy looking mats when framing artwork.  In addition to the 2 printed parts a single edge razor blade, a #4-40 x ½" long screw and a #4-40 nut are needed. An M2.5 screw will probably also work, but the nut might spin in the hexagonal hole in the back.

To assemble:

1. Put the razor into the blade holder
2. Insert the blade holder into the slot in the base
3. Push the nut into the hex shaped hole in the bottom
4. Thread the screw through everything

The attached files fit the razor blade tightly with my setup.  The blade holder may need some adjustment to fit the blade tightly depending on your slicer and printer settings.  

There are notches in the base to help align your cut, but the relative blade position changes relative to the mat thickness, so make a practice cut and adjust your measurements.  I found a few light cuts to work better than trying to cut the whole thickness all at once.