Super simple runout sensor (1.75 mm)

Use with a standard large microswitch, see pictures. Remove the roller if there is one. Open the switch and bend the buckling spring, to make the switching a little smoother, requiring less force.
Use two M3x20 screws and two M3 nuts to secure the switch to the case. Feed filament through the slot and align the case so it goes through smoothly. 0.1 mm or finer print detail definitely recommended.