Hey Forrest,
here you can see how different materials trigger the inductive proximity sensor Omron E2B which was available at my local CNC equipment supplier.
Source: Omron E2B Inductive Cylindrical Proximity Sensor Datasheet
Some people also use microswitches but I would always prefer using inductive proximity sensors because the contacts will not wear out or attract dust.
The problem to solve anyway is how to attach them. As far as I know, everybody I know used 3D-printed parts, as Tom @TMToronto did as shown above. Thankfully Tom published the 3D files in this thread! An alternative would be milling them out of wood using a CNC (less plastic). 