Convert the geometry to an Editable Polygon or add an Edit Poly modifier. Then go into Polygon subobject mode and select the top polygon. Inset it 17.5 units.
Extrude the inset polygon 100 units.
Switch to Edge subobject mode and select all but 12 white edges as seen in the image above.
We have just opened our tutorial directory! Feel free to submit tutorials hosted on your website or other online tutorials that you think are worth mentioning.