This is a small, inexpensive CoreXY mechanism which illustrates how CoreXY works. When the user turns the two knobs in the same direction, the X axis moves. When in opposite directions, the Y axis moves. This is purely for illustration purposes, there is currently no attachment for a pen or pencil (nor laser nor extruder). However, the concepts learned from this can lead to scaling up and improvements and the addition of stepper motors.

Required:
4 x 3mm by 100mm aluminum rods (or other material may work)
1m GT2 timing belt 6mm (this is overkill, a thinner belt should work)
6 x 3mm x 25mm (or longer) screws
To print:
1 x Inner bearing
2 x Outer Bearing
1 x Frame
4 x Spacers
8 x Spindles
2 x Control knobs
Optional:
2x Belt Connectors

I used PETG but any material should work.
I used 20% infill but use 65% to be sure, it doesn't take much longer to print.

Here is a video of it working: https://youtu.be/tElMsCfxDCs