As part of the upgrade of the Pierre Auger Observatory, called AugerPrime, scintillator detectors will be mounted on top of all water-Cherenkov detectors (WCDs) of the surface array. By combining the data from WCDs with those of scintillator surface detectors (SSDs), we can derive information needed to reconstruct the energy and composition of cosmic rays at energies higher than those reached by the Auger fluorescence telescopes. In this contribution, the mechanical structure of the SSDs and their optical properties are discussed. We also present novel techniques used in the construction of the prototype detectors deployed in the Engineering Array. The efficiency and light output of these detectors were measured using cosmic-ray muon tomography, providing an absolute calibration of the detectors at the same time. We have achieved $\pm10$% uniformity in the signal for particles impinging on any given detector at various impact points.