This work reports on the design, construction, calibration, and validation of a low-cost, clip-on extensometer for crack mouth opening displacement (CMOD) measurements in fracture mechanics specimens. The device employs readily available components and integrates a full Wheatstone bridge with instrumentation-amplifier conditioning to achieve high measurement resolution at minimal cost. A dedicated calibration protocol, combined with the compliance-based framework specified in the standard, enables direct conversion of bridge output to CMOD. The device’s performance was verified on compact tension specimens through a monotonic test, and cross-validated against a finite element simulation and analytical displacement prediction. The results demonstrated linear-elastic agreement within experimental uncertainty across a verification window, as well as a reproducible CMOD–time histories suitable for both toughness evaluation and crack-growth assessments. By providing full documentation of the hardware and manufacture procedures, this study facilitates rapid replication in contexts where commercial instruments may be cost-prohibitive. Furthermore, it reaffirms that CMOD measurement using clip-on extensometers remains a robust and essential technique, complementing modern optical methods.