This makes cutting a v-groove slice out of foam core easy. Why do this? So that you can bend along the notch to put a nice corner in the foam core without cutting through it.
This object is a small tool made to hold a small section of utility knife blade. It is shaped and sized so that the inside edge of the tool is the middle of the v-groove. When dragged along a ruler edge on foam core (5 mm or 3/16" thick) it will make a cut at a little more than a 45 degree angle. Use it again on the other side and it'll cut out a nice V, and the foam core can be bent and glued to make a 90 degree angle with a smooth and unbroken outside edge.
In other words:
1) Draw a line where you want the groove (i.e. on the inside of where you want to make your bend.)
2) Hold the ruler against the line and use the tool to slice along the ruler's edge. This cuts one side of the V.
3) Do it again on the other side of the line to cut the other side of the V.
4) Peel off the cut segment, then fold and glue your foam core.
You should be able to print it without supports in the orientation shown (i.e. upside down.)