This is an openscad project so you can easily customize it to your needs. The included stl file has following dimensions (base-width, height): 70x100mm It has holes in the middle of each edge of base and at the top so you can use a piece of string to explain cross sections in a pyramid.

This project can also be found on github: https://github.com/adamwalach/openscad-projects/tree/master/geometry-teaching-aids